Most dogs learn Down very quickly, but of course, some dogs don't.
It's a very vulnerable/submissive position, so some dogs really don't like it.
I second the crawling under the leg technique, I believe that's called a "bridge".
Also, you can just "shape" the behavior. Give her a treat for just touching her nose to the floor, then only treat if she goes a little lower, then a little lower than that... if she's jumping back, it sounds like something is making her uncomfortable. How's your body language?