Okay first of all I would need to know how old the girl in question is, Unfortunately, girls in some age groups can be ficle and love one person one day and another the next...I know, I was one when I was a teen. K lets assume that you are a teen, or even in college. The first thing I would do is to listen to her feelings, most guys are so interested in what they are doing and trying to be cool they miss the signs that the girl will give them. If she is cool to you, then turn on the charm, first of all find out what SHE likes to do, and go from there. Also if she really doesn't like the same things you do..you may wanna rethink this whole thing. You really do have to have something more in common than sexual chemistry. If all else fails, be a hopeless romantic, cause that will get you further than anything else. Take her for a picnic in the park, or if you have an apt. of your own, buy a major amount of candles, wine, and a nice dinner, (if you can't cook, if ya can, than cook for her women love that) set it out in your apt. like a picnic, with candles, maybe some flowers for her (psst. find out what her fave flower and meal is) and have it brought in, or you cook whichever, she definitely will find this very romantic...oh yeah and don't try to jump her right away, serve her dinner, and a nice desert, like chocolate covered strawberries...why them, well if she is pms'ing.you have the chocolate angle covered if not, she'll love ya for thinking such a romantic thing, or buy a fondue pot, and do a chocolate fondue for her with lots of candles, and fruits, pineapple, strawberry, mellon, etc.
Or if your relationship is that progressed, buy her a promise ring, make sure it's at least 10k with a nice little diamond in it. First try the dinner, if she acts uninterested, well then I'm sorry but she's just not into you. If her feelings have "atrophied" as you say usually she has found someone else to fulfill her needs.
Good Luck to you.