Ok, so here it is:
first possibility (probability = 50%) she's probably self-concious about her vagina and doesnt feel comfortable with you getting down there. A lot of girls feel uncomfortable with how their vagina's look or b/c they are sexually inexperienced. However, some girls who feel this way are usually more eager to go down on their guys to make em stop bugging them about going down on her. Yet on the other hand, going down on him would make him feel waaay to guilty and he'll wanna go down on her back (unless hes a selfish health questions, which, lets face it, happens often). You will notice this if she reacts abrubptly wny time your head travels anywhere near that area. This is the scenerio where you are more likely to get her to go down on you before u will get permission to go down on her. This takes time and assurance that she is sexy and beautiful and pretty much a ton of complimenting. Also, when she finally allows u down there show major enthusiasm so she has a great time and knows you're enjoying it as well. Most girls, and many guys can only be fully satisfied when they know their partner is also satisfied. When she is more comfortable with you and sure that you will accept her body the way it is she will almost definitely go south on you as well, because she will want to make u happy and won't have that insecurity holding her back.
Second possibility (probability = 50%) she is nervous about her oral capabilities, and doesn't expect u to do her the favor either because then you can pressure her into returning it. This may be b/c she is inexperienced or she was traumatized/embarassed by a bad oral experience (you know, unable to satisfy the guy, was too inexperienced, had a rumor spread around that she was bad, etc etc etc). In this case you let her get really comfortable with you over the next few weeks and if everything is going well, then you can gently bring up the topic. Let me give you some golden advice that even some women won't admit to: we are very co-dependant and you can get us to do pretty much anything if you make us feel needed...Like you can't do something without our help/approval. This gives the woman the impression of importance and control. So tell her that you really need her to talk to you (p.S. This convo should happen when u are completely alone but not having sex cuz she wont take u seriously she'll just think you're horny). Stress that she is doing you a favor and it means so much...Fail-proof. Then just be honest about how this bothers you and how she would make you so happy by doing this and you want to make her just as happy...You get the point. Once she agrees you have to be ultra-supportive and tell her she was great to build her confidence back up.
Third possibility (probabilitry = 10%) she may honestly just not like oral sex. Not b/c of some trauma or insecurity (those can be overcome with time, patience, and affection to build up trust and familiarity), but b/c of a more solid, immovable reason, such as religious/cultural restrictions, penis phobia, compulsively hygenic, or some deep personal revulsion against the act. In such cases, while rare, don't even bother trying to change her min cuz it ain't happening. Take it or leave it.
Good luck dude...Hope u figure this one out!