im not so down with people who view oral sex as whorish and yet expect me to give them oral sex...so i geuss she asumes you're her whore?
I found it kinda disgusting when I did that the first time. The guy seemed to expect of me, to take his whole dick in my mouth, which felt really uncomfortable. So to start with, and get used to being down there, maybe she should just start kissing you down there, and as she gets more and more comfortable, she can start licking, and then finally sucking... Give her the time to explore you down there, to get to know and like the feeling being there. make sure you showered before, that gives a lot more confindence. And, if she's not willing to give that pleasure to you, don't give it to her.
If she is anything like me or my girlfriends she will lose her hangups once she does it once. My first time I was so apprehensive but I can tell you that I took to it like a fish to water after that. Nothing is so intimate as taking your boyfriend into your mouth and swallowing. Hope she gets over it and the two of you have some great fun.
give her time to get use to the idea and start off slow. Constantly asking and forcing her to do that will obviously not make it happen any sooner, so just go slow. It's good though that you two are actually talking about the situation and trying to solve it :0)
The first time I just didn't know how to go about it, so I kind of just put it off. She might be worried about disappointing you, I know a few guys who can't come from oral and it makes the girl feel like a dissappoinment to them. Just tell her how good it would make you feel in a non forceful very sweet way. The worst way to go about it would be to act sleazy, because than she might put your dick in her mouth, but I doubt you'd be happy with what she did with it. One of my friends at work bit a guy(quite hard) because he tried to get her to swallow him when she didn't want to.