Well, my first instinct would be to say that she does not enjoy performing oral sex for one reason or another. If even the promise of reciprocity is not enough to encourage her, then there is a reasonably profound reason she turns it down.
My g/f is sort of in the same boat but claims to enjoy performing oral sex, however, has yet to complete the act - much the same as you described. Turns out she is just a selfish lover. Now to see if we can rectify that
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