Dealing with a Refusal
What is really going on when a trained horse refuses to jump?
If your horse is refusing to jump, then he or she might be lacking in respect or trust for the rider. Or, even with all the respect and trust in the world, if the rider is not setting up the approach well enough then perhaps the horse is just saying "I can't".
If a horse does not have ample respect and trust for the rider then it just can't realistically be expected to go anywhere and do anything that it doesn't really want to do.
If you know that you and your horse are in a reliable partnership and you think the problem is solely regarding the jumps, then the next question would be is he or she sound enough to be jumping?
But many times, it just comes down to the rider's experience and ability.
Taken From Horse to Horse