I agree with britt, I kiss and squeeze with my legs when I want my horse to jump. Try putting a pole a couple inches in front of the jump. Maybe you said that it was 10? Inches. Wasn't sure if that was the height..
If you do decide to try fillers, if you can't find small enough ones, try buying small fake potted plants or large fake flowers and just tuck them under and decorate it however you like. It's always fun :P