There are many reasons a horse may not make much of an effort. Maybe the jumps are too small and they are boring him to death. Maybe the tack doesn't fit and it hurts to jump, maybe he just doesn't LIKE jumping.
Horses are like people. Different horses like different things. Not all people like doing math problems. If asked to do them, they may not perform well.
I would try doing gymnastic jumping grids (google it) and see if you can't exercise him and give him the power to pick up his legs.