I think you should ask her, that way you'll know exactly what the problem is and how you may be able to solve it. There are several reasons I can think of as to why your girlfriend may not like it...
She may simply have a really sensitive gag reflex or not enjoy the feeling of something long and thick down her throat blocking her airway. Not sure how many of you men are aware of it - but it can be very difficult to breathe while giving BJ, esp if deep throating. Using more licking and hand/stroking action, only giving a shallow BJ but using the hands to work the shaft and balls...
She may have had negative experiences with BJ in the past, perhaps being forced to do them, having her head held down until she chokes, etc - in that case make sure she knows that she has complete control and more importantly give her complete control, don't push her down, no hand on the head or holding/pulling hair...
Do you give her oral, purely to make sure she'll return the favour or because you actually enjoy it? If she feels that you're only going down on her so that she'll suck your cock - then that just perpetuates her dislike of the act...
It could be the taste or texture. Are you trimmed, shaved or waxed? If not, try it. Make sure you're clean, using a non-scented soap. I've also found from experience that guys who smoke and/or drink lots of beer and/or eat lots of junk food taste really bad! Try pineapple juice or cinnamon - it really does make a difference, even just lots of fresh fruit.
You could try to make it more interesting with whipped cream, chocolate, etc. Not just on your cock, but to make it more fun everywhere! I think one of the tricky things is that if you push it too hard, she may end up disliking it more. Women - we're tricky creatures sometimes...
Hope that helps, but I think the best thing is to ask her. And if the sex is great anyway, is it important?
xxx