I have 14 hens & 1 roo in a 6 x 6 coop. 10 are the older flock by 2 & 4 weeks, then there is the 5 younger. After I got them merged, they still stayed separate. I made a roosting bar, 3 rungs, all they use it for is to hop onto the 2 ft wide shelf that is 3 ft high off the ground.
The 5 younger ones used the shelf right away, the original 10 puppy piled on the floor in the corner.
This went on for 2 weeks, then Light Dawned On Marble Head....
Maybe it would be better if we were up there too. Now all 15 are up there, and the 5 younger, cram together in a corner of the shelf or the olders will pick on them.
I had the nest box in at the very beginning, and for a while they would go in & out and poop in it, but never to sleep. Still thinking about putting a second shelf below the top one, so see if the youngers will use it & have more space to spread out and not get picked or pecked on.
Is your nest box removable? If they are not laying yet, take it out for a while & see if that forces them to use the roosting bar. I also heard not to use the dowels because their feet are exposed to the cold, where as the 2 x 4, they can tuck their feet under them & keep them warm.