You have one of three things going on:
1. Feather picking
2. Rooster damage
3. Bugs
I am guessing #1. Find the culprits and put some pinless peepers on them or cull them.
Sorry this is a sore subject with me because mine look the same way. I have a feather picker or pickers, and I am getting ready to hunt mine down and remove her/them from my flock permanently. I, too, have addressed the nutritional issues, the boredom issues, the stress issues, and still my hens look terrible. I have searched for bugs numerous times, and found nothing. I got rid of my rough roo, and still the girls look terrible. I am now dedicating my chicken observation time to locating the picker(s), tagging them, and removing them. I think you might consider doing the same.
Good luck.