Well, it could be the way you are sitting on the horse, yet it could be just the horse. If it's the way you're sitting, try to adjust yourself and not put to much pressure on the inside leg. If its your horse, then it is a simple fix, yet still a hard one. What I do is keep the horse on the counter canter, but just slightly hold his head to the inside. The counter canter is extremely uncomftorable for horses. If he still does not want to change, make smaller and smaller circles until he does. This will work for teaching him lead changes, as well as getting him to want to take the correct lead. Hope I helped!