Well, different people have different ways of getting off. Some erogenous zones work more or less for each individual. I know a guy that can only get off if his nipples are stimulated. I'm not sure if it's actually a physiological issue or if it's more of a conditioned response. But from what you've said, it sounds like a physical thing. One suggestion I'd make for you to make things easier for you (so you don't have to bend over funny to reach her butt) is to get some toys. Anal beads can be fun and she can pop them out as she wishes, or a vibrating egg, so she can get constant stimulation there.
I wouldn't be too worried about it. Just relax and enjoy. The more relaxed you are, the more relaxed she'll be, and the easier to orgasm it will be for her. Just support her and don't treat her like she's weird because of it, have fun with it. :D