She may have insecurities, unsure of how her body looks or smells, or whether or not she's "normal" down there. Fear of judgement can be a huge mood killer.When I was growing up I was never told there was anything wrong with me, but discussion of bodies was rather taboo. It took MANY years before I was comfortable with myself. Also, if she's ever experienced any trauma, that could make her less comfortable as well.

Are you guys able to have discussions about intimate matters? What turns you on, what kinds of touch, how and where? She may or may not know why she's not comfortable with it, but communication is so important for the health of the relationship. Patience, kindness, and to be supportive and non-judgemental of each other goes far with intimate matters.

Also, sometimes there are health issues that may escalate after oral sex, e.g. cystitis