What made you so sure that it's her and not you that's the problem? There's a pretty real possibility that you suck at sex. Also, the fact that you even asked this question tells me that you have a cursory understanding of female pleasure. Any modicum of research on the subject will tell you that many, many, many women don't "get off" from sex. Expecting them to cum in the same way that men do (as in there's a slow build to an identifiable climax), is a mistake, because in expecting that, you're letting yourself down. It doesn't happen often. Here's how a lot of women experience sex. Listen up, penis-havers of the world: It feels good. Like, really good. Great even. But it's not necessarily climactic. It's like a high, sucstained level of pleasure that doesn't increase or decrease appreciably as the sex continues. Like a massage. It feels good the whole time, but there's no point in which it feels the best toward the end, and then you roll over and fall asleep. This isn't to say women don't get off. Many women have climactic, orgasmic experiences during sex. But if your girlfriend isn't getting off, it's either because she is one of those women who likes to fuck but doesn't necessarily orgasm, or it's because you're not fucking her in a way that makes her cum. Either way, you can't avoid talking about it. Ask her if she likes what you're doing, and if there's anything you could do more or less of, or do differeently. You can also try different positions. The femael G-spot is on the front wall of the vagina, and any position that's from behind increases the chances of hitting it. Also, get her riled up first. Take time to kiss her, caress her, go down on her…foreplay the shit out of her. Make her want it.