I know that I've had some bad experiences when it came to receiving head:
1) I'm very sensitive down there, and some men seem to want to go all crazy on me!!! I don't like my clit sucked on and so much pressure applied to that area. For her, it may feel that way too. Feel her out and see if she likes it slower and gentler.
2) I cum easily, and I know, if a man is doing the right thing down there, I'm going to cum too quickly. So, I like switching it up...
3) Some men just aren't good at it, and it feels like a waste of time...for me. I know he may be enjoying it, so I'll do it for the sake of his enjoyment. But, in all honesty, I may be thinking, "Please...no more!"
4) Just like some women may give a half-ass blow job, some men give some half-ass oral sex...they're not committing to it and a woman can pick up on that...
C/S