She'll eventually do it if she wants to. Don't ask so much, just be in the moment. At least you're having sex, dig? Enjoy what she brings to the table instead of what you feel is being left out.
The case I'm about to mention is an exception, so don't make a move like I did unless you're REALLY sure.
So, anyway I had an ex who didn't like giving head. That was a little weird for me. Not really a deal breaker though, because she was good at other things.
Well, I started noticing more and more that she liked things rough, and loved to talk dirty - an awesome surprise for me, because those are things I love to do. I'll be honest, I first started dating her just for the looks - HB8 with a face to match. I didn't figure her to be extremely apt in mattress gymnastics, because in public she's actually a very traditional type (Eastern European, very family oriented, etc etc). That, and she was a really slim girl - 5'4", maybe 105 lbs.
First, she started with biting and scratching, which was awesome. Then the dirty talk, which was great. Then, she started asking me for more and more crazy things, which I obliged, happily.
After I noticed the patterns, I was like "damn, this girl is up for anything" so I just took charge. Started probing here and there with fingers and making suggestions. Anytime she didn't stop me, I just kept going.
Turns out, she was, as Archer put it, the "Pele of anal." This girl LOVED getting it where most girls would smack you for even asking. In the shower, while playing video games, it was a marathon. Good times!
Finally, when I remembered that she didn't like giving head, I decided to, instead of asking for it, just shoving it in and doing my thing. She liked that much more than having me ask for it.
Once again, VERY specific situation. I just thought I'd share in the sense that you have to feel these things out (and sometimes ask questions) and then, most importantly, take action, whether that's banging more of a specific way, or finding another girl.