Spend some time with her. Does she make you want to do things with her (beside bang her), do you enjoy talking to her (about things other than banging her), do you think about her all day (besides banging her). Take her out and show her a good time (besides banging her).
If you do spend some time with her, do you enjoy it? Or does she just annoy the crap out of you? Do you value her opinion? Do you apply her suggestions to your life or do you include her in your life? Do you share secrets with her or do you trust her w/ your secrets?
Are you interested in more than a fuck buddy relationship w/ her? Is she w/ you? That type of sex can be very impersonal and some girls just can't deal w/ that aspect of it. The expectations of the situation become different and one party can become resentful when both parties don't have the same view of what they want out of it. (Did you ever see the movie "Fatal Attraction"??)