Mites will suck the life right out of a chicken. That should be first priority. Dust with a permetrin power. Concentrate on vent and under the wings, but make sure it gets down to the skin. If she has mites, then the entire flock should be treated along with the coop/run. Discard/burn all bedding and spray the coop with a permethrin based liquid - measured correctly. Get all nook and cracks and allow to dry before allowing chickens back in area.
Also, she needs to stop all food except her normal feed. She can wet it to make a mash if the chicken is not interested right now.
If she were my chicken, I would start her on vitamin therapy to help her get over this issue. 1/3 super b complex and a vit e capsule with a little bit of egg to help with the uptake of the vit e. I would do this until she is feeling/acting normal.
If the mites are causing this then without these steps for the mites, the chicken has little hope of recovery.