Maybe trying to get commitment. Some girls use their vagina is a tool for that, rationing it out as your behavior dictates, as a reward.
But, I don't look at these situations like that. There was a girl I was sort of hooking up with, we'd go out, make out here and there, an HB8, and seeing for about a month. I had her in my bed, even, but could never close because of the amount of resistance she put up. She had a boyfriend at the time, and I got the feeling that she wanted me to gf her - not something I wanted - she was branch swinging.
Anyway, what did I do? I stopped talking to her. I met other girls and hooked up with them.
When I meet resistance to my advances I quickly lose interest. If she wants to keep me interested she's going to have to learn to give me what I want, and sooner rather than later, otherwise she can find herself someone else to try to manipulate.