It looks like there is plenty of light in there with that window, they should be able to see to go to bed when it starts getting dark. That's probably why that one hopped down when you put it up there, it could still see well enough. They can maneuver in fairly dark conditions better than some people give them credit for. Even if you are in a cold climate it's unlikely there is a constant cold wind whistling through there to stop them from using that roost, I don't think that is the problem.
Chickens aren't that great at reading a tape measure, most don't even have access to one. When you set them up there is sounds like most of them spent the night, it may not have been that crowded. 12" from the wall is a standard recommendation but I have seen a few sleep in strange places closer than that. If they want to sleep there, they will orient themselves in a way they can. Still, if it is close it may be uncomfortable. That looks like a 4" x 4" roost, they should be OK with that but it does look closer than I'd put it. I know I'm rambling on this, I don't think it is a problem, but maybe you could drop a couple of hangers from the ceiling and install a 4' long roost further from the wall if you are uncomfortable with it.
I don't see anything really wrong, I think they just don't want to for whatever reason. I'd go in every night after dark and set them on the roosts. Be consistent, every night. I think you can train them to sleep up there.
Good luck!