I'm in the same situation with several of my girls. I even separated the roosters from the hens for 4 months. Didn't work cause then the hens started acting like roosters; fighting, etc.
My worst bird, I made a saddle for - mostly so the sun won't burn her - and she's been wearing it for 6 months now.
Still, no feather growth even though several birds have gone through molt.
I dust her constantly, even tried separating her from ALL the birds for a three month period (six months ago, back when I pulled the roosters out). NOT ONE FEATHER tried to grow back in.
Is it possible that the skin becomes so damaged that the feathers WON'T grow back?
She's very lively and lays eggs daily. She's a red star; about 3 or 4 years old.