Like ALexandra and the rest mentioned either she had a bad experience or either she is not feeling comfortable. Never pressure a woman to enable you to commit oral sex. Sex and it's variety of ways of performance should never be committed when there isn't 100% consent between the partners.
Relax your needs and make sure she is relaxed as well , let her trust you with her body. Let time do it's job and in no time you will find yourself between her legs.