Sometimes it's hard to try to guess what is going on even when you are looking at them. I'll throw out a possibility or two. That does not mean any of these are the problem.
It is possible for a flock to have a disease and be able to give that disease to other chickens, but the flock has developed an immunity to that specific disease. You will never see any signs of that disease, no matter how long you quarantine them. It is possible your old flock has a disease they are immune to and have given it to the newcomer. Your old chickens may be the disease host, not the newcomers.
There are different possibilities as to what this disease might be, but I'd consider coccidiosis, especially if you have had a rain or a wet spell lately. Chickens can develop an immunity to coccidiosis, especially when they are chicks. There are different varieties of the protozoa that causes coccidiosis. Immumity to one of the protozoa does not give immunity to all. Wet weather increases the chance for coccidiosis to be a problem. So that is one possible scenario.
I'll also mention worms. A chicken can handle a small worm load without a big problem, but if the number of worms in the hen's system have built up to a dangerous level, it could be the problem or weakening her enough so something else in conjunction could be causing it. A lot of the time, it is not one thing but two or three things in conjunction that are causing the problem.
Is she egg bound?
Since she doesn't roost, is she broody? I don't think so from your description, but I'll ask a dumb question. You never know.
This one is a bit of a stretch, but chickens can be pretty vicious to others that are really low in the pecking order and to weak chickens, especially on the roosts. I've had hens so vicious and brutal toward younger chickens weaned by a broody that they quit roosting in the roosts and find a safer place to roost. It is possible that the hen is weakened by disease, injury, or someting wrong with her internal organs and the other chickens are picking on a weakened chicken. So she is not roosting and they may have picked off some fearthers.
Have you checked for mites and lice? You need to check at night in case it is roost mites. Those hide during the day and only come out at night. If the hen is weakened a bit from something else they may be affecting her more than the others.
It is quite possible ot is something totally different going on. I really don't know.
Good luck!
I feed them layer feed with oyster shells and grit on the side.
