Why are some women so reluctant towards receiving oral sex?

I (20m) have noticed with the last couple of encounters I had that the girls (19-21) all were ok with me stimulating them with my hand and ok with sex but gave negative body signs when I aimed towards giving them oral sex (they pulled me back up and kissed me or stuff like that, so it wasn't too negative). My question is just why? I thought girls always complained about guys not wanting to eat them out and then the one guy who passionately loves eating out is not let.

Edit: Thanks a lot for all the answers, helps a lot. I guess I'll work on trying to make my partner feel more comfortable to avoid her feeling self-conscious or uncomfortable. And yeah, I got it, taking :D

But seriously Thanks :)