Hmmm....well I waited mine out and she seemed to grow them back just fine.
I was paying extra attention to any feathers around or how the others were treating her..She also did have a minor case of mites and lice, which I treated with Poultry Protector. Worked great.
I would say as long as your not seeing any severe redness, bleeding or sores to may be give it some time??
Is everyone drinking and hydrated?
Getting their vitamins and nutrition?
I ask only because you said it's 7 out of the 8, which indicates whatever it is, it is affecting everyone.
Have you checked the other parts of their body for the lice and mites?
Although Gilly's feather loss was in the rear, I found the mice and lice were mostly behind her neck and around her crop area.
Hope this helps a bit.