Hold her with her on your lap and look at her skin on her back. Pull the feathers the wrong way and lift them up so you can really SEE the skin. Check around her vent area and look at the skin on her belly.
You can see the mites, they will be crawling on her, they are small and tannish clear colored, but they are big enough for you to see with the naked eye. Also, you may see some feathers on her underside and around the vent with a white crust near the bottom of the quill. Those would be louse eggs.
There is a mite that can be very destructive to poultry and to some people. They are also known as chiggers. These are little red mites that come out at night and feed on the birds.
I am thinking it could be a few things. 1. The mites/lice if its bad enough, it could have caused her oil gland to become infected or at least inflamed like it is.
2. picking by other chickens 3. Something else, maybe a rodent attack? but that's just reaching.
Check for the mites first. You probably wont see the red ones, but the dusting and cleaning you have done should take care of any mites/lice.