a girl could have 1000 reasons to not put out. maybe she isn't interested enough, or maybe she is too interested and doesn't wanna screw it up, because you are her first. if you are serious with her, don't screw it up. one time i have waited for a girl for 3 months (in the meantime i had sex with others of course), and when she finally was ready, she was very devout and nice girlfriend who did everything for me and turned out quite the little wild thing in bed. she was reluctant at first because she had no confidence and i was her first, and she needed time to trust me 100%.