Pain is the most common cause of bucking. Usually saddle pain or rider caused (snatching his mouth, sitting down to early, bumping with spurs), but with jumping you need to also consider leg pain. Have him gone over by a vet and farrier.
Does he only buck when you ride or does he also buck for others? If you lunge him or free jump him does he buck? How do you ride differently to and through a grid?
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse