Need more info. Is the horse worse on one lead? Will he pick up both leads easily on the flat. It could very easily be a training issue. If the horse is too far on his forehand he will never be able to successfully get his leads.
It could also be an issue with hind end fitness. Hill work is your answer there.
It could also be a soundness issue, but assuming the horse is fine with simple changes and shows no other issues, I would assume it is a training or fitness issue.
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