He is most likely "skipping", it's when the horse takes a trot step to change the lead because they can't do a flying change. My trainers old horse skipped in flying lead changes all the way up to advanced 3 star level just because he simply couldn't do it. So I can't help you with how to fix it, maybe canter-halt-canter across the diagonal. I beg to differ on Kristys view on cross cantering though, if your horse is truly at the Grand Prix level in his training maybe you just arn't riding the change properly he could be leaving his hind end because he isn't properly collected. I would explore the idea of taking some clinics with another trainer and see if they can help you.