What you are talking about is discipline not ability.
Should every horse be able to take a fence... Yes. But not all horses are physically or more importantly Mentally able to.
I rode a very lovely TB mare that had failed out on the track because she was strong willed and didn't particularly like the gate. She was pushed too fast when they attempted to make her a hunter. And now she is hit or miss on jumping. You demand she says no... ask nicely she might jump.
I worked with her for a full year, had her coursing 3' but she was never consistent. She would just stop and shut down over fences. A lot of people would say take a crop... the horse has to want to do it, you can't force them.
Like they say you can lead a horse to water, but you can't make 'em drink. Same goes for any other aspect of riding.
If the horse does not like to jump, why force it to? Especially when you don't own it. Why not look for something that enjoys it's job.
The mare I rode, she's now schooling 4th level... seems we had a dressage princess on our hands. She doesn't balk no matter what you asked there.