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!