Dan Savage would say that it could be from jerking off. If you pull more than get laid and I assume you do (no shame), you have trained yourself to come in a certain way. Cunts don't feel the same as a hand, it might not be easy to adjust.My boyfriend ahd the same issue when we stared dating. We decided to give it some time and I worked on my PC muscles, now he gets off regularly form intercourse and if not, that's ok too. Intercourse alone doesn't always get me off, why should it for him.
Savage Love has done plenty of columns on the subject. Here's a link
http://www.thestranger.com/current/savage.html