Do you lunge Obie before you ride him? Maybe he isn't stretched out enough or something? I would lunge him over the jumps until he goes over them every time.. Does he only do this to you, or does he do it with other riders also? If he does it with everyone, maybe he has an injury that nobody is aware of, or, maybe he just doesn't like jumping. A lot of horses just don't like doing certain things, and when they get their mind set on something, its hard to change if for them.