During the beginning of my relationship with the GF, there was an abundance of, admittedly inexperienced, oral sex all around.
As things got forward, we realized I was the (much) more dominant figure in the bedroom, but due to my inexperience I didn't really know how to handle the whole thing.
I really enjoyed taking control of the whole blowjob act, but that unavoidably led to gagging and messy situations. Thinking I was hurting her, or causing a lot of discomfort I ended up declining her advances and just having regular sex instead.
She found herself in a similar position with yours and ended up asking me about it. I reluctantly explained myself, and after she assured me that nothing was happening that she didn't feel comfortable with and we agreed on some basic safeguards, I'm proud to say I haven't declined a blowjob in 8 years.
I'm also not proud to admit that the frequency has plummeted. But that's neither here nor there.
A couple of years ago she started stopping me from going down on her. So it was my turn to put myself in your shoes and ask what the problem was.
She said she didn't feel comfortable, which to me sounded stupid after 6 years of relationship, but what could I do. I kept trying though and sometimes I managed to get things going and get her off. Other times she stopped me, telling me I wasn't doing it right.
And here I am, 27 years old and 9 years with the same woman and learning to eat pussy again. Figures.
So if my experiences mean anything, here are a few possible causes: