Usually problems like this are unintentionally caused by rider error, a green horse, or some sort of pain. we would need more details to offer any substantial advice for this issue. But I would first make sure all your tack fits well and nothing is hurting the horse you don't realize. You have to find out what's causing the inconsistency before you can fix it. He could also be over faced by the height of the jump. It could really be a myriad of things.
When approaching a jump make sure the rider is seated with constant pressure from the leg. Make sure the horses chest is at the center of the jump.
Flat work. Get your basics solid!
Grid work lots and lots of gridwork