This could be part of the problem. That coop doesn't look big enough for 5 full-grown chickens. You don't want your chickens getting in the habit of sleeping in the nest boxes. That will result in dirty, poop covered eggs. Recommended roost space is 1 ft. per bird, recommended coop space is 4 sq. feet per bird. What are the dimensions of your coop? What kind of winter climate do you have? I would definitely take out the heat lamp. I'm sure it's making it way too warm in there. Also, it doesn't look like there's much ventilation in that coop. Very essential for happy, healthy chickens in the winter.
I'll do measurements tomorrow and report back. Probably not enough room. I keep the heat lamp off but it's there in case it gets below 20 degrees. Usually it's about 30 degrees at night in winter. Maybe you're right the upstairs of the coop doesn't have enough ventilation. It's strange that one of the five sleeps in the coop and the others on roof. We installed panels above the coop so they are protected from snow but not from drafts on top of coop. Thanks again