Typically a good jumper will run out or refuse because of rider error. Considering he seems to pick the simple fences, I am going to guess you may "quit" riding to this fence, maybe drop a shoulder or loosen your leg.
Primo is the jump anything from any distance, angle, speed horse... but if I drop my hand in a line, he will take the easy way out instead of over.
Work on making sure you're not jumping up, leaning or dropping support. Basically ride like it's a big, funky looking vertical and see if it makes a difference if it makes a difference.
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