Well, the advice everyone here is going to give you-- and rightfully so-- is to talk to her about it.

Aside from that, what are you doing leading up to sex? I mean, if you are getting to her place expecting to screw and putting no effort into seducing, flirting, and otherwise wooing her, it's hard to expect her to be interested. Treating every time with her as if you were trying to have sex with her for the first time may very well be what she is expecting/hoping for. If, on the other hand, you are doing these things, then things get more complicated.

My wife has had a low sex drive for all 14+ years of our marriage with the exception of last three months of her second pregnancy, so my experience is that if a woman has a naturally low sex drive, there isn't much you can do about it. It may be biochemical, it may be stress, it may be something else, but sometimes a low sex drive is mostly out of your hands.