Soundness issues aside- I agree with those posters who say this horse needs to build up the correct muscle. they all seem to have a favored side or lead.
the current mare i am working with, have been for a little over a week - she was very spotty in regards to her right lead. it was a mix of muscling but also “i can’t” … you could watch her trying really hard - she knew the request but just couldn’t quite figure out how to do it- it would be like me trying to pick up a pen and write with my right hand. I know how to write, i just can’t make my right hand do it.
so finally she started doing it on the longe line - getting praise and being left alone to canter when she got it correct. No nitpicking- just get the lead right. If she didn’t i’d bring her down to trot right away, let her relax and when the time was right, ask her again.
Slowly but surely, she’d start getting it.
Then under saddle we went through the same process. I’d ask her, if she picked it up wrong i’d either bring her back or let her go and just change direction so it was correct- and then do a proper downward transition to trot, change direction and ask for the correct bend, and then ask again. No nitpicking for her to get fussy about - just bring her back and put it back together, ask again. I could tell and feel in her head she was trying - you could feel her go for it, and then at the last minute she’d take these itty bitty steps and swap it, and then get flustered - her head would come up, she’d start tossing her body around, etc. So it took a lot of patience and consistency, and honestly positive support and encouragement for her.
Once we had a quality trot again, i’d make the circle a little smaller (she’s teeny tiny- like 14.1hh) got the bend correct on the smaller circle she’d get the right lead canter under saddle. Once she got it i’d just let her go, let her relax into it, and when the moment was right, bring her back down. As a rider you could feel the correctness of the bend, the feel on the outside rein felt right, her hip was aligned, voila.
She still can’t really pick it up correctly on a straight line, but on a 20 m circle she’s accurate 95% of the time, just a week later. We’re chipping away at it, and with more consistent work she’ll have it in no time.
I am sure others with much more experience will chime in too. this has just been my experience with this particular horse. Good luck!