I think it's an interesting one. I went off sex with ex-hubby, because I felt like he was an older brother or something. His approach to sex (the same routine, same moves, same time of day, no variety, not respecting my requests for something different) really turned me off, and even though he always got me off it started to feel like he was only engaging with the animal part of me - making me aroused by giving me head or whatever then fucking me, so I'd be turned on in my body but not in my head. And I felt rubbish about that and the more he pressured the more I got annoyed, but not once did we manage to have an adult conversation about this.
My most recent ex stopped wanting to have sex with me - my libido was higher, and he still turned me on and I wanted to explore more kinky things in bed which intimidated him so it created a vicious cycle which made him feel he couldn't give me what he wanted in bed and withdraw, and I'd pressure him more and more. It was one of the reasons we broke up.
My current partner is someone I have crazy hot sex with. Interestingly, we were both in relationships where we stopped wanting to have sex with our partners and relationships where they stopped having sex with us, so we know our triggers, turn-on and turn-offs and can talk about this and communicate when something is wrong. His biggest turn-off is feeling he's pressured to have sex, especially at bed-time. Mine is lack of sexual variety. So we're non-monogamous and have sex at whatever time of day or night he feels like; both of our personal turn-offs sorted.
You need to find out what are her turn offs, be willing to take on criticism and make changes but DO NOT make this out to be just your fault. As a woman, the trend to blame men for everything that's wrong in a couple's sex life makes me mad. She's just as much to blame but treat it like detective work: both of you, together, are trying to find out what is going on.