Could it be that you feel like she's just taking more than she's giving? I don't have a sexual experience regarding this but I was making out with some girl one weekend and it was getting pretty heated but then as I was feeling her up she wasn't returning the favor, I couldn't even guide her hand to do that. She was into it, but just unwilling to do anything for me. I ended up just stopping, making up some excuse and leaving.

Just a thought. I'm not suggesting you just get up and leave, but maybe consider that she's taking more than she's giving. It can leave you feeling underbenefitted, and studies show that people are always happy in an equitable relationship.