Hmmm I hope that she's doing well. Maybe she's getting picked on when you're not looking or maybe she's being chased away from the food and just having a hard time b/c of that???
I had a chick that was having all her tail feather being pecked out, so I removed her to the brooder with the Cochin chicks (she was the big chick on the block in there lol), and a different pullet started resting a lot and acting sickly. When I separated her, she perked right up after just a few hours b/c she got some sleep/rest, a good amount of food in her and plenty of water. She was feisty once she was feeling better. I put her back in with the other chicks and they didn't bother her after that. Now, the one with the pecked out tail feathers is back in with her brood mates and doing fine.
Also, I've had adult chickens in the past that were simply deprived of food by the rest of the flock and that stressed them and compromised their health such that they couldn't fight any infection or whatever and I lost her (that was Beauty).
Maybe, if she does poorly with the flock and does well when you separate her, then maybe she needs a smaller flock of nicer chickens to hang out with. Maybe just a single friend, mostly what was suggested to me in that case was either a very nice chicken or one at the bottom of the pecking order. (I don't, personally, like the idea of using the one at the bottom of the pecking order.)
