Issue with Left Lead Canter

So, my horse won't canter on the left lead, I'm pretty sure it's my fault, but I'm not sure what I'm doing.
Some background info: when I bought him he would canter both directions, no problems, but about 6 months ago he just started refusing to canter on the left lead. I specificly remember the ride when it started, right lead was fine but when I asked him to pick up the left he would just speed up, I tried several times, making him slow and then asking again, and when I forced the issue he gave a small buck, which he had never done before. I managed to get him to canter, didn't care which lead, and left it at that for the day.
Not long after I moved him to a new agistment, and whilst lunging him a couple of days later, noticed his left hind leg seemed to be locking at the stifle when cantering left. A few days later I lunged him in the round yard and he was fine.
Since then I have been unable to get him to canter on the left lead. In the round pen he will pick up the wrong leg at first but will then change over. Under saddle... no luck. I've been working with my instructor on this and we have managed about 3 times to get him to pick it up, but only after a bit of a struggle. As soon as he feels me asking for a canter he will flex to the outside which causes him to lean in and I'm afraid if I force the issue he will fall.
Then, yesterday I let another girl who agists at the paddock ride him, and asked her to try him on the left lead to see if a) he would pick it up and b) if there were any problems with his left hind leg if he did.
He picked up the wrong lead as expected and she brought him back to a trot and asked again, same thing. So I told her to push him and see if he will change leads of his own accord. And he did! Three times she got him to canter, and even though he picked up the wrong lead to start with, when she used her inside leg, he corrected himself and changed leads. And his leg looked fine, no locking or stiffness, which was a huge relief.
However, I rode him today and had no sucess, so it HAS to be me hindering him, I'm just unsure how.
His tack fits correctly, his saddle was fitted when I first got him and was re-fitted 2 weeks ago, and the fitter remarked on how well it fitted him. I've had a massure out to him twice, and although that was months ago, I'm fairly certain that pain is not an issue and he does not show any signs of pain when I put pressure on his back, plus he got the correct lead with a different rider.
When riding, before I ask I make sure he is flexed properly, I usually ask him on a corner, and that I have him nice and forward, and when I ask I slide my outside back behind the girth and nudge him, asking for flexion with my inside rein and keeping him steady with my outside, using my inside leg to try to push him out, but he tends to ignore that. He refuses to flex to the inside properly and if I try to flex him properly, while at the canter, he will just lean in and cut the corner, until we're practically circling on the spot.
What am I doing wrong?

Sorry for the massive post!

This is very common. My reiners do this from time to time. It takes about 1 day to fix. Walk him in a circle to the left and just move his hips in but don't make him pick up the left lead. When he moves his hips in, take your leg off and then repeat. He has just become dull on that side. I do this with all my reining and cutting horses about once a week just to keep them soft. Eventualy you can push their hip in and leave it in all the way around the circle and going the opposite dirrection. Hope this helps

It sounds to me that you are unbalanced yourself and due to this, you are getting the answers you are recieving.

Another reason, could be that you are not keeping him under himself during the transition, and not lifting him up into your aids. Asking at the correct moment, keeping his left shoulder under himself and lifting him up.

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I am sure Spyder, Anebel, CJ8Sky or Koomy will beable to give you great advice.

Make sure that when you are asking for the left lead canter you aren't dropping your left shoulder to the inside. Stretch up tall through that inside shoulder, push his haunches to the left and then ask.

I had a horse that was very diffcult to get to pick up the left lead.
Once you rule out any poteinal vet problems and have talked to your trainer there were a couple excersices that I would do with my old gelding.

The first I was always working on bending and flexing him, and working on shoulder fore and haunches out.

And once he got the hang of that and was lose and supple we'd get him to do a shoulder in and ask for it then. It usually worked.

And circles circles circles. They were my best friend... and they worked to loosen him up.

The one work out that worked the best is Id get him on the rail, do an turn on the haunces, ask for a canter for 4 strides, stop, another turn on the haunches canter for four strides and so fourth.

If you can keep your horse in control and in good form it works really well, the horse starts to memorize what lead he is supposed to pick up in which direction.

It is really a common problam. When ur picking up the canter do it in a coner so that he's bending and turn his head stightly to the inside and lean your self just the slightest bit to the inside.

You answered your own question when you said he would switch the the correct lead when inside leg pressure was applied. Someone mentioned earlier that you may be unbalanced, which is very likely. I would guess that you sit heavy on your right side and right leg. Through this, your horse has become "stuck" in a slight right bend, with his barrel logded to the left. It could be so slight that you can hardly see it. With the barrel in the way, its hard to move his hind left forward, which is why he looked stiff in his hind left from the ground. To fix this, fix yourself. Start with riding at the walk and drop your weight in your left stirrup and left seatbone. Use left rein until you feel a shift under you and feel softness in the rein. Release the pressure, it should return shortly. Repeat the cue. If you feel through your reins with your reins even, your left rein will feel heavier. This means he has a slight right bend.

Long term signs of this problem are usually lamenes in a hind leg from over bearing the weight. You could also have a vet check out to see if there is another reason that the horse is shifting his weight that doesn't have to do with you, but if there no other signs of lameness, then it is usually rider related. Once you do "dislodge" him from the left, he will be unstable and bounce from side to side, as a rider, you need to steady him. You will need to repeat this at walk and trot and it will take a while to change his pattern.

Update: I have managed several times in the last couple of rides to get him to pick it up. I think a part of the problem was that I had a bit of a death grip on my outside rein, as I was trying to prevent him from tipping inwards. The other day I bit the bullet and let it go and with LOTS of insde leg... viola! Left lead canter! Its is FAR from perfect, he still leans in and is a bit unbalanced, but I think this is partly because he is under-muscled in this side from not using that lead in so long. But, we have the lead, so now we can work on improving it!
He is also quite reluctant to flex correctly to the left (and has been for a while) although he is better after a long, gradual warm up, so I'll be getting a massage therapist out to see him as I think he may be sore along his right side. If he's not she may be able to point me in the right direction as to where to go from here, so we shall see.
One lady at my paddock asked me if he was raced in NSW, of which I am unsure but it is possible, as they race on the right rein, which could explain it somewhat.
Anyhow, thank you all for your help and I will keep you updated!

MiEventer - his balance is an issue, but I have a feeling his balance issues stem from something else. He doesn't like flexing to the left, so on the right rein he will flex right which throws his balance off. I don't know... sore back maybe? Yet his saddle fits great!...

Ahsisi - thank you! I will remember that!

Silentbravery - yes, circles are our best friend too, lol! Lots and lots of circles and bending.

Ivorygold - I only ever ask him on a corner or if he is flexed properly. Still doesn't work :(

Flitterbug - yes I do sit heavy on the right side, as I try to keep him from falling to the inside (left). Thank you for your post, I will try this in future!