My sister was like that when she first started fucking her guy. She would come all the time when they first started having sex, then her orgasms gradually tapered off where she didn't come very often. She said it was all this built up tension she had from not being fucked for a long time before they started having sex (even though she masturbated all the time) and when she got fucked often for quite a while, that tension got released and she calmed down. If her guy gets frustrated that she doesn't come, he'll just lick her clit until she has a really good orgasm-it always works. Me, I always come, regardless of how much sex I've recently had.