Rhole Island Red is lethargic, as in a trance.

My 3 year old RIR hen, Florence, who lives with two other hens, started acting lethargic about two weeks ago. She pecks occasionally, but doesn't appear to be eating much, and has runny poop. She has been treated for mites, and I had to bathe her butt once to remove poop from feathers. She stands in one spot for literally hours, and last night, I had to lift her into the henhouse at dark, as she was standing at the bottom of the ramp.
The other two are just fine. They are all locked up at night, and free range during the days. I clean their roosting spot daily, and put down fresh bedding. The coop is quite clean - no smells. I look, but have not seen any trace of worms. I feed them chicken feed and seeds, and let them into my garden occasionally, but in the summer they are more interested in grazing (lawns, small woods).
What can I do for my chicken? Any suggestions? My local vet knows nothing about chickens.
Worried in Michigan