This is interesting, although I'm not sure I can help you much, I have a similar situation and share your concern. My BO, Henrietta, is a beloved pet, and she's fading before our eyes.
She's now ten months old and extremely skinny from a long spell of broodiness during which she was also terribly besieged by mites. We got rid of the mites over three weeks ago and she's in much better condition, (redder around the face and comb and with a lot more energy) but still rather disinterested in eating. She will catch bugs (although she gives almost all of them to her chick) and eats tons of grass and greenery, but is almost totally disinterested in other types of food. I haven't felt her with a full crop since the hatch. It's always either completely flat or just a little grainy, with a few sunflower seeds in it (which I feed to her by hand, as it's pretty much all she will eat, and only then if it is right under her nose). I'm worried that if she doesn't start eating a bit more soon, we'll lose her.
Thing is, she's always been a bit of a fussy eater, and I suspect she's had something not quite right with her all along. She laid lovely medium eggs, five days a week, before she went broody, but never really got plump and filled out like I thought a laying hen should. (In contrast, her coop-pal Bella, a Marans hybrid, is totally solid, and eats like a total pig. If anything, she's overweight!)
I wonder if some hens are a little anorexic, for no particular reason? It's sad, but perhaps they're never going to thrive, no matter how much we cosset and coddle them.
