True what monkey says. If it happens, it gotta be all about her! period .
I've had some dealings with this issue and overcome em all, and the best tip I can give you is to play with her erogenous spots. (if you dont know these spots,
http://www.freewebs.com/xfman/s3sex.htm Xfman posted a link to some of them. dont use the lower parts so much, basically cause if she has jeans or anything the like, it might ruin the spell shes under a bit)
Gentle caressing starting with just stroking her arms, dont stroke with palm, use finger tips and as you move up or down , move them like a wave(like if you gently were to caress her body with a feather). After a while you can start moving around to other parts of her body, like go from her hand move up to shoulder, neck lips, neck, down towards her breasts , half moon circle and then other arm, moving down to belly in a while and the line of her pants. Go ontop and rub inner thighs etc etc. You get the point in taking one small step and gently escalate, at any form of hesitation from her part, just gently move on to a safer zone but come back to that spot she resisted with not to much time in between.
The basic idea is to always touch skin when you can, so you get her nervs tingling with excitement. Note that some girls may say that it tickles, then just say that its because that they dont relax enough and move to safer zone and then back again.
Also, since you are sonstantly moving your hand, the sensation will not be the same as if you would just place your hand there. Just placing a hand on 1 of her breasts might invoke discomfort, while when your moving it, she enjoys it on a whole different level, and she will be waiting for your hand to get back, so that she can feel that great sensation yet again. This will make her comply when you start going for the "BIG" zones. If you move on to her breasts and are gone before she has the time to say something, a part of her will have accepted the fact that you were there, and she will be more easy to comply the next time you get there.
Dont go down on her, atleast not at first. Just gently rub it down there and work ur magic. When you get her hot enough, she will say the words you are waiting for.
If we jump a bit back to the start again. When you start moving your fingers on her arm, smalltalk all the time. just talk about anything really. keep her mind a bit of from what you are doing and let her get accustomed to the feeling. When you start hearing her breathing really calm in deep breaths, you know that its working and you will se that you dont have to talk either, all her mind can think of will be your gentle caressing.
This sort of push/pull has never failed to get a girl aroused. Just be sure to only use this if you gonna give this girl the night of her life. Virgins should be shown some respect ... even by PUAs