It might be a hymen problem, like Mal mentioned. It can be thick, too low, any number of things that makes any kind of penetration almost impossible-been there, got the t-shirt. (luckily or not, my first time was with a guy who didn't really give a rat's ass and after a few half-hearted tender seconds, rammed-end of problem
) hopefully, you're a bit more considerate.
Couple of questions: When you have tried with fingers, is she flowing at all? moist? dry? Dry will prevent insertion and cause pain. If everything's nice and wet, it is probably a hymen issue.
Here's a suggestion that works wonders, specially if she might be on the pill:
Buy a spermicidal suppository product. See if she can insert it. It will lubricate quite well and not be as icky as K-Y. If she can, you will most likely be able to follow
If not, she needs to talk to her doctor, following Mal's advice.