Here are some photo's of my horse, one of his shoulders is slightly more dipped than the other, also I think he has slight muscle wasted, I have been working him more lately and it's not as bad as it was and now have a different saddle.
If you girth it up tight enough to mount and run the flat of your hand down his shoulder between horse and shoulder is it tighter on the nearside than the offside? Measuring from inside the pommel back to his withers (with your fingers) does one side sit closer to him than the other? Do you have difficulty getting a canter strike-off with a left lead?
Any of these might mean the saddle's dropping in too far on the 'hollow' side. I'd usually adjust the flock to level the saddle up, but you can get pads which allow you to shim one side thicker than the other. A flock adjust is better, IMO, but shimming will probably do the job.
But if the horse seems happy in your current saddle and you've not noticed any recent change in behaviour, sensitivity in the saddle area or rub marks, I wouldn't bother. He may be perfectly content as he is :)
Not tried measuring inside of pommel will check that tomorrow!
But if I remember correctly the offside is smooth even pressure, but the nearside isn't quite as smooth. Also I notice when riding his back of nearside shoulder seems closer to the saddle than the offside? So I'm guessing you could be right in saying the saddle could be dropping a bit...
I'd like to have it flocked adjusted but it's hard to get saddlers where I am, hence why thinking of a shimmy pad to test for a while.
He goes fine in the saddle, no problems, his right lead is his bad side so hard to comment on the strike off as just been sticking to walk/trot work the past few months
Sorry - just noticed my second line should read 'between horse and saddle', but I guess you'd got that :)
If you get a shim pad it's usually a bit of trial and error deciding how much extra you'll need under the nearside front of the saddle to keep it level when you're riding. It depends on how the padding (usually closed-cell foam) compresses.
I find where people sometimes go wrong with these pads is to pull them too far forward so they stick past the front of the saddle like a saddlecloth. The pad really needs to fit so it's in line with, or just behind, the front of the saddle. It looks wrong, but then the padding sits behind the shoulder, where you need it.