bare back flock

Looking for answers, please help. Flock of 25 hens and one araucana rooster that will be a year old end of this month. In December they started losing the feathers on their backs and around their butts. The couple white leghorns have managed to retain all their feathers. The rooster looks horrible, he has no feathers on his entire back half and his wing feathers look like something has been chewing away at them. All chickens seem to be strong and laying okay but look horrible. I have inspected them for parasites but to no avail.
3 days ago I introduced12 new hens and yesterday 5 new big healthy roosters. I figured my araucana would be done for because he looks just awful. When I went to check to see if the new roosters had attached him, I found most of them either hiding or on the top roost. When I got them out from hiding my scraggly araucana pounced on them, running them back in hiding. My point, he looks absolutely terrible but he is stronger than the nice healthy new roosters.
If it was the molt I figured they would start retrieving feathers by now.

PLEASE HELP!!!