Looks like a nice set up that chickens would like. I have 3 ideas, just taking a stab at it. I know it's not the biggest deal ever, 3 of the 5 not using the roost but it's sort of fun to try to figure these chickens out.
1. Could they be having a hard time flying up the 2 feet to get to the roost because it's close to the ceiling? When they fly up, do they have enough clearance for their wings? Or maybe the feeder is in the way when they try to open their wings to fly up?
2. Is it hot up on the roost being near the ceiling and in a relatively small space so those that aren't high enough in the pecking order to get a seat by the vent choose the floor? Do you close the door (I think it's the red framed opening on the left in picture #1) at night or is it open/screene
d shut and if it's open, do those that sleep on the floor sleep near it so they can be closer to the cool air?
3. Are the 2 that use the roost higher in the pecking order. Do the 2 knock the other 3 off the roost? Would you say that the 2 that use the roost sort of hang out together and the 3 that don't sort of hang out together?
Just trying to think like a chicken. There must be some reason the 3 sleep on the floor because chickens really like to sleep in the highest place possible.
In my case, I have an 8 foot long roost that is a 2 x 4 set with the 4" side up. The roost is 4 feet off the ground with steps leading up to it. I have 8 chickens that are about 27 weeks old and 4 of them roost on one end of the roost all crowded together and the other 4 roost on the other end all crowded together. These 2 groups of 4 often act as 2 little sub-flocks during the day. But, every day, they get more and more integrated and just recently, they are all mixed together in a line on the roost as 1 big group of 8. I notice that their integration increases as more of them start laying. I've read that this is the case, that the flock becomes one as they all reach maturity.
Anyway, hope this helps you figure out what those little chicken brains are thinking!
Guppy