I think you are having some Last minute resistance (LMR) or maybe you just need to push the right buttons.
Remember girls have this anti-slut defense, maybe she just wants you to take control and lead the interaction toward sex (use kino escalation to reach the point where both of you can't stop and just want to have sex)
Check this two links that might help you:
(Kino Escalation) Vin Dicarlo Escalation ladder e-book:
http://www.scribd.com/doc/16768511/Vin- ... ion-Ladder
Last-minute resistance:
http://pualife.webs.com/s2lmr.htm