Some horses just don't want to do it
My MIL used to have a beautiful steel dapple grey Irish Sport Horse named Trueman.
He was exceptional in the Dressage Ring, wonderful out on the trails, but a hellian in the stadium ring.
No matter how much work she put into him going over a fence, it was a fight every single time. He was a nasty dirty refuser, and even if he went over a fence once, didn't mean he was going to do it again.
He was even that way with trot poles.
It was like he was absolutely terrified.
My MIL used to have to beat his buttox everytime to get him over a fence, and when he did, it was totally like a Carasaul Horse, and terror was in his eyes.
Now, he was a completely different horse over a CC fence. He loved it.
She admitted the fact that he wasn't going to be the Eventer she wanted. She sold him to a Fox Hunter, and he is very happy at the barn down the road, Fox Hunting.
Not saying that this is the OP's case - but there are horses out there who are like that. If they don't want to do the job they are being asked to do, they will tell you *not all, because most just comply with their human rider and do as they are asked*
And I wasnt only refering to my horse I ment in general how do you go about fixing them...
You firstly, want to rule out pain. Medical Veterinarian examination, xrays, eyes, teeth, saddle fit *this is through a professional maker/fitter*
Then when all has been ruled out, I would turn to the rider secondly.
Then if the rider is ruled out, I would turn to the horse and the will that they may not have to do the job, or the fear - for example:
Some horses just learn to not jump due to error occuring over time and time again *aka catching in mouth* or *hitting the fence and the fence hurting them* *fence falls on them* etc, etc - whatever the reason may be - try to figure it out.
And sometimes, the horse just doesn't want to do the job that it's human rider wants to do. That is when the rider has to accept the facts, and either do the job the horse most enjoys, or find a new home for the horse and get a horse that is more compatable for them *the sport they want to do*