That could well be Mareks (or the other thing that's like it, which doesn't have an easy name). Sorry to say but if it is, she's unlikely to get better.
I lost a few to it a couple of years ago but my flock seems to have settled down and become immune now. It helps if you breed your own replacements rather than buy them in, if that's possible (even hatching eggs, as they gain their immunity as little ones and seem ok, whereas if they catch it first time as pullets they seem to go down with it more often). Or if you buy commercial pullets they're often vaccinated.