To encourage eating I've made some scrambled eggs, dripped on the vitamin E oil, sprinkled a tablespoon of powdered vitamins & Probiotics, & some mealworms, sometimes add a bit of chickweed or pecking greens if I've got any growing at the moment, if not, a bit of a cabbage leaf I've picked apart into little pieces. Not a morsel is left behind. When I had a few going through grumpy molt & not eating as much a few days, but not really ill, this did encourage them to eat & get some vitamins.
She sounds like she's got something going on, not well. I've had occasional mites or lice with just some birds over the years, & one thing in my cases...I noticed that not all of the other birds had them, only ones not feeling well. Did mites cause illness or did illness encourage outbreak of mites since birds wasn't well & didn't dustbathe or preen? It could be either, mites will drain a bird, though & can make them sick. Even if no other birds sharing space seem to have mites, do clean entire area & treat everyone before they get worse. Then try to get this hen to eat & figure out what is ailing her. Poor thing, best of luck with her.