Have you checked them over real well for mites and/or lice?
Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.
Part the feathers right down to the skin around vent, head/neck and under wings.
Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.
I've had some bare butted girls that had no bugs, some had very red skin that lasted for either just a few days or a few weeks but eventually went away, the feathers grew back after the molt. I don't worry about it too much unless I see bugs that need killing then use a poultry dust with permethrin, feathers still won't grow back until a molt tho.
This makes no sense at all.