Maybe every time she changes on her own get her to do a flying change and do a really tight circle so she has to balance herself and then go stright and if Sheba changes again ask again for a flaying change or even a simple change and do the small circle. Keep doing this exercise on this reign and the other to make sure she is even on both sides. It might be just a learn habit. Also gets chiro out to see or a vet if you think it needs attention
My OTTB does the same thing, I would check her back hind stifle or SI joint -it could also be weakness (I'm going through all of this myself:))
She might have a sore stifle
She canters on her left lead just fine. It's only going to the right. No matter how long I keep the bend she switches it out and counter canters. And whenever I try to collect her and get her to stop she gets mad.