losing feathers

if there are no signs of any creepy crawlies (check the feather quills to see if any eggs or adults are on there also check around her vent) then she could be going through a moult.

We had a hen recover from an illness and then started losing feathers like you'd never believe. She would get up and leave a ring of feathers in her wake. I think it's as a result of having been ill (if you've checked for creepy crawlies and not found any).
It will be a shock to her as well, so give her some poultry spice in her food and some tonic in her water to help her through.  It sounds as if you're doing it anyway but keep an eye on her just in case there's  something more sinister going on.