Is she able to masturbate herself to orgasm? Why don't you lube each other up and then mutually masturbate to orgasm, this might give some clues as to what gets her off. Generally, it's a lot easier to make yourself come than to have someone else do it for you. But some women can't even do this, if they're really inhibited about sex and/or so inexperienced they don't even know how to masturbate, they'll have trouble getting themselves into a frame of mind where they can achieve orgasm. And, some women simply don't come easily and require hours of hard work, some women come easily and freely every time.
What I find works best is Chase's advice on eating a women's pussy... start with a long tease (say 3 to 5 minutes of just kissing her inner thighs, pussy lips etc, licking her vaginal canal and so on, getting gradually closer to the clit... preferably, tie her hands up first, because she may get impatient and start masturbating, also she may not realize you
do actually know where the clit is located)... then
commit to the clit and just eat that fucker until she comes, this can take half an hour or more, so you have to be prepared to have a pretty sore tongue afterwards. I find there are quite a lot of different motions I can use, directly licking the tip of the clit fast with the tip of the tongue is one, another is to make slower circles around the clit and hood which tend to gently move the clit from side to side, with minimal contact with the clit itself... another one is to pull the clit into your mouth and repeatedly push it against your top lip in a kind of biting motion (except you're using a lip and a tongue rather than both lips or your teeth)... experiment with this until you find something that gets her excited and keep doing it, you'll want to be listening carefully to her responses (but some women give almost no response to anything, in which case you just have to use your judgement and persevere as best you can). If she seems to be getting excited then
for fuck's sake don't break rhythm, just keep doing it! If the excitement seems to subside then this might be a good time to try something else though.
Also, bear in mind that women require different kinds of stimulus in order to come, there's no "one size fits all" formula. For instance my wife couldn't come without some penetration, and I noticed that when she masturbated she'd use both hands, one hand inside and one hand rubbing the clit. If I ate her clit for a long time nothing would happen, but then if I inserted a finger or fingers and did some "come hither" while eating her, magically she'd get all excited. You can also try fucking her hard with your fingers while eating her. Or just the time honoured finger job, while nibbling on her ears and breathing heavily in there and kissing her deeply, you sometimes don't need anything fancy, just the finger will do the trick. If you're really having trouble, see if you can get her to masturbate during sex... doggy style, or if you kneel beside the bed and fuck her from there, she'll have easy access to her own clit and will be able to get herself there. Plus you'll get incredibly horny watching her get excited and you'll get even harder
Another option which works really well is to use a vibrator. Even a hard case will definitely be able to come using a vibrator, properly handled! Look up the "orgasm torture" if you want to make it more intense. Anyway, get down to the sex shop and pick up a good vibrator if you don't already have one. Every seducer needs it in his toolkit. Just remember to put a condom onto the vibrator before you use it each time.
I'm not super good at making her orgasm through vaginal intercourse, but you could also try reading the article on adapted missionary and see what you can make of it. I enjoy adapted, but she doesn't come.
Hope that helps.
Ray