My girlfriend has problems giving me head?

We've been together a while. She has given it to me, but its always short and she never offers it, I have to ask. She always does it for like a minute and then stops. She's never given me an with it. I'm wondering how I can get her to give it more and for longer. I eat her out almost nightly and always give her an in the hope that she'll offer to return the favor, but she never does. And then often when I ask her if she'll do it later that night, she agrees to it and then when the time comes, she doesn't say a word about it and then when I ask about it later, she says she forgot or was too tired or something. What can I do?