Ask her! Maybe there's a reason she's not totally comfortable with sex; maybe she's completely comfortable and just hasn't been in the mood, lately. There's any number of reasons why your girl is shy about sex with you, and the only way to figure out why is to see if she'll tell you.
That being said, a good component of sex with most women--not all, but a great many of them--is foreplay. If you and she are routinely sexually intimate, try including more foreplay than intercourse in order to get her engine revved. As a matter of fact, it's not a bad idea to have good, old-fashioned necking sessions: Just make out with no pressure or plan to have sex. Those can help make her feel comfortable and aroused and may possibly lead to sex at a future date.
Kissing passionately, kiss her on the ear, and then move down to the neck. Caress her breasts, etc. Pull her very close to you and hold her tightly. If she's responding to your moves, then she's horny. Kiss your way down and kiss her body. Start fingering her while you're kissing her. If you're both comfortable, you can do oral on her. If none of these work, then she's either too uptight, not ready, or is just not attracted to you.
You should first use the term ''make love'' instead of ''sex'' when you're talking to her about this.
When the perfect time has come, you first start gently kissing and touching her and tell her how beautiful and special she is. This will make her feel more comfortable. Most of the time you will see her reaction. If this is possible you can go on.. Good luck and do not rush things!
Make it all about her! That's the most generous thing you could do. Tell her you want to give her pleasure and know how to give it to her (do your homework first). IF she's not ready, hey she's not ready. Or she might just not be ready for you. Either way, don't pressure/manipulate her into sex. I know its really easy to when you're not getting any and your hormones are going through the f'n roof. But is it right? Be patient, thoughtful, respectful. Also, be a man. Take control and initiative. You're the driver just don't drive her off a cliff by pressuring her too much. Finally, don't be too much of a "good guy". Chicks say they like good guys but really don't. What they like is a surefooted guy that can protect them and take care of their needs. A gogetter who knows what he wants and knows how to give her what she wants.