I see there have been no posts here for a while. I have the exact same symptom on several of my hens. They have been getting Layena daily, clean water twice a day, get outside when weather permits etc. I don't think I could feed them better. They also get grit. I have not given them any vitamins or anything like that.
Mine look exactly like the photos on the initial post; feathers missing, red and a few sores. I'll get some seven today.
I need to add, no mice or rats and we have 20 hens and two roosters. They are 9 months old. Only about 5 or 6 of the hens have this issue. Would lice or mites not be on all of them?