Some girls are very very self-concious about being eaten out. It's a psychological thing. One girl I dated was so afraid of having a guy tell her one day that she smelled/tasted like a fish, that she wouldn't ever let any guy go down on her, and it took me like a month just to make her feel comfortable with it. Also, because of the psychological barrier, getting her off oraly took way longer then getting another girl off.
A female friend of mine doesn't like getting oral sex because while it feels good for her, she can't get off that way. On the other hand, she gets off easily through vaginal intercourse. Usually when guys go down on her, they get impatient and frustrated about not being able to get her off, disappointed, or are very stubborn and ruin the mood by insisting to keep going. She avoids all that drama by avoiding oral sex.
My advice is, if she doesn't want you to eat her out, respect her wishes. If you keep insisting, you will annoy the sh!t out of her, and will come off like you are trying to hard to prove yourself. Even if she lets you go down on her, she'll have all these psychological barriers in place and wont be able to enjoy it.