Try setting the jump against a fence and set some pole bending poles on the other side to close off any routes she may take to get away from the jump
I think the most important thing is to assess your own riding.
Are you riding forward to the jump?
Are you relaxed and in a good position?
Are you looking up?
Are you aiming for the centre of the fence?
My own horse is sensitive, if I am not confident coming into a bigger fence, he will not jump it.
Make sure you are confident and focused on the jump. Don't tense up or hold your breath.
Maybe lower the jumps and school her over those; then gradually raise them?
And as always, have her teeth, back, tack etc checked. Pain could be a prevalent factor.