H-scenes are replayable when you unlock GF mode aka when you finish their storyline. You'll be able to do all the scenes in various other rooms and in front of other girls too.
At what chapter is gf mode?
After chapter 10, aka the end of the story.
I'm trying to complete Juliet's chapter 10, but nothing happens. I've got 3 stars on her gifts and ive hung out with her several times
Might have to trigger it in a specific room or a different time, a handful of chapters are like that.