BlueMouse, I didn't notice the earlier post so I'll ask here. Is she eating & drinking like the others? Have you tried giving her treats (mealworms, fruit, etc) & see how she responds to them? Checked her over for mites, ticks, fleas?
Is she running around outside of the coop, resting in the shade, scratching at the ground - basically acting like a normal chicken otherwise? Is your rooster (if you have one) paying her extra attention, or another hen driving her off constantly?
How does her feces look? Watery, mucusy, what colour?
She seems to be eating and drinking normally, foraging around with the others/dustbathing/acting like a normal chicken. Certainly convinced she's dying when I feed in the evening at close up time and gobbles right down. I call her over for bugs when I garden and she zooms them right up. She's one of my only two adult hens, and the other one is broody, so she's looking a little rough since she's the only hen available for cassanova to express his love (except if he can catch my 18 week Cuckoo Marans, which isn't often.) She lays me a jumbo egg every day. She's certainly not getting picked on since she is the undisputed queen of the coop.
I picked her up the other day to have a look over because she was looking a little thin. I was alarmed by how prominent her keelbone seems, she looks almost triangular from the fornt if you know what I mean? I gave her a good check over, didn't see any kind of mites or fleas, I checked the bases of her feathers, nothing, checked her face for ticks, nothing. Had a good look at her vent, no smell, vent seemed clean. This hen has had an on and off poopy butt for the last six weeks or so, I just assumed that it was just sort of part of having the extra poofy buff orpington trousers, since it didn't seem extreme, and there's no foul odor/whiteness around the vent like in gleet. Her droppings seem normal to me, greenish brown with a white cap. She does throw of cecal poops, but I think that's relatively normal?
I only feed once a day in the evening, and not a whole lot, but they have four acres of free range available, including almost an acre of yard. Everyone else seems in good condition. Their crops don't seem stuffed when they come in at night, but neither does anyone else seem to be losing weight, and the younglings are growing apace (well, Except Eliza. She's still relatively the size of a powder puff, but then again she's a bantam.) I'm just confused. Heck. Even my broody looks like she's in better condition.