Whenever I have a problem like this in my flock that I find difficult to see any good reason for, I plant myself where I can watch them. In observing the action during the time of day a certain problem seems to be occurring, I usually very quickly understand the "why" of it all.
Roosting problems can be caused by any number of things. Many of my roosting problems were caused by poorly designed roosting perches. If a perch is too high, it caused problems for heavier breeds such as my Cochins and Brahmas. If a perch was too short, the chickens were reluctant to hop up because there wasn't room to flap their wings to get the lift required without knocking others off the perch, resulting in conflicts.
So, my advice is to watch them at roosting time to see what's going on. Since they've established the habit of sleeping on the floor, try placing them on the perch and then watch what follows. That ought to give you some clues.
