Hens bald spots on back

[av8rwife]

So far i've seperated the roos from the hens, haven't seen any lice or mites, the egg production is down. What other things could be wrong with the girls?

 

[KelsHapyChicks]

I noticed that 3 of my 13 hens (one year olds) had some thinning feathers on their backs across the saddle. Then one of the girls became more "raw" looking at the tops of her wing area. I put Rooster Booster product on it thinking that if the roosters were being too rough, this product would discourage the pecking. When I looked closely today, I noticed that there was a bald spot (not raw, just bald) underneath her left wing. What the heck. Are the other girls pecking her out of boredom? She is a Barred Rock. Gentle and easy going. Any ideas my chicken lovers? I do not see any evidence of mites although I did dust her with Diatimatous Earth. They have regular access to dust, dirt and ash baths.