Some horses are just awkward. My horse, can lope like a million bucks, but he can't jog to save his life. He's got a fierce long trot, but no matter how many things I try, his jog will never be pretty. So the exact opposite of the horse you're working with. If you make the lope become the focus, it will improve. It might never be perfect; some horses just aren't good lopers. If the focus hasn't been on the lope, he's not gaining the coordination nor the muscle to do it. Simply trotting, though, will not cause a horse to lope poorly.