Op,
When my XW and I did have (very very occasional) sex, it was always at night, always in the bed, and always followed the same pattern.
This frustrated me hugely. I was, er, "up" for sex (sorry!
) in the morning, afternoon, evening - whenever, basically, and keen to move things away from the "usual fayre"
Unfortunately she wasn't.
I found out much later that she was (sadly) "ashamed" of her body - the fact that she had gotten older, things were headed south, she had put on weight etc etc. The thing is, none of this bothered me, but it was enough for her to feel much less confident about herself, and only feel comfortable with sex at night with the lights off.
Is it possible that your partner is feeling the same?
Maybe take a bit more of the lead on things. Instead of just talking about it, why not instigate some sex in the daytime?