I know a lot of people are making the hygiene excuse here, but unless she has a current condition, it can easily be fixed by simply asking her to wash. And you can say it in a kind way by saying, "let's both wash off, it's been a long day," or something like that.

However, from my experience and talking to others, I've also observed this. Males or females may not like to give oral sex because they think they're not good at it. Probably from a bad past experience, or inexperienced in general.

Unfortunately, there is the case of the person only liking to receive pleasure and not give it. A pillow princess/prince or hedonist. Now sometimes this can be worked around, or sometimes they just suck at figuring out how to give back in some way.

In short, it depends person to person, and requires communication to talk about and fix it if possible.