All you can really do is be patient with her, and make her happy - yes, happy in the bedroom, but also by doing little things for her to make her just happy in the relationship in general. In turn, she will want to make you happy, and when she is ready, will probably want to make you happy in this way, as she knows it is something that you would really like to do.
I also think she is probably just scared to do it too. She's probably never done it, and feels like she doesn't know what she's doing, and is afraid of not being good at it. Once she does eventually do it, make sure you help guide her and also tell her how good it feels. Once she sees how much it turns you on, she will want to come back and do it again.
Or, are there certain things she won't do with a partner until she is married? Everyone has their own limits.