same deal with my gf. She oragasms if I finger her, and of course she can by masturbating. But with intercourse, no. Although when we make love, she never wants to cum. Even if I pleasure her before we have sex, she always stops me right before she orgasms. She prefers intecourse much more and says that the constant pleasure is way better than a one time burst. Which I have to agree with that. And if my gf did cum during intercourse, we'd have to stop for a while since she gets pretty sensitive after an orgasm. Does this happen to any other women out there?
Instead of trying to make her come, I'd say try and pleasure her better during intercourse. If she comes fine, but when my gf and I make love, thats what it is, making love. Not masturbation. I remember when we first started having sex, she complained that her previous lovers always tried to make her come, either by trying to last longer, starting and stoping, or just trying to go real fast. Neither really come close to real pleasure.
We dont really experiment with different positions, we sort of jsut go at it and do whatever we want. But we could try the coital one, I know manly is a good source of info.