Marin's new scene involves sleeping while wearing something you got from her earlier.
What I initially thought but 6 times not a charm, I restarted a new game and tried again still no.
It worked at the 8th time on a New Game+ don't know how or why?
Maybe you weren't wearing all pieces? Or slept in the wrong bed?
Could also be there was something actually wrong and the NG+ fixed it, I suppose.
