Do you have anything she may have eaten in the last few days that might have no agreed with her? check her poop. Thats part of what separating her can do, help you isolate things like poop color and knowing who it belongs to. If its a contagious illness, she is putting the others at risk being housed with them. At this point, stress can often cause some serious issues, so isolation in the dark is a huge relaxing factor for her, sleep wont hurt her and dark and quiet will help until you have something to go on.
Check at the base of her feathers, look for mites or thick dander. The base of the neck, under the wings and around the tail are classic areas for infestation. She might have a mite problem. Permethrin powder will usually take care of these.
If her poop is bright green, she may have ecoli. Stumbling and loss of balance, unable to stand can be associated with botulism or hopefully no Mareks.
I just noticed that you said she is a polish. Polish do not ahve a full cranium like a normal chicken. their skull is open on top. As a result they are susceptible to knocks on the head that can make them go wonky. I had a silky that happened to, she got a good peck on the noggin from one of the other birds. It took a while, but she did recover, though was still wobbly for a long time after.
Try getting some yogurt down her, plain unflavored, no sugar and NO sugar replacements. Just natural. If she wont take that, you can use asidopholus pills crushed up. Fine powder is easier to get inside the mouth than something wet that might choke her. It might be something as simple as a tummy ache or a load of worms. If its worms bothering her, getting some dewormer can help. There are tons of articles on here on how to worm chickens. Acidopholus is natural and wont hurt her, its found in yogurt so it will be OK with other meds.
See if you can get some Corid or Amprol from your local vet, liquid is best if she is drinking. If she has coccidiosis, these meds can really turn her around.
Has she been laying? If so, it may be egg binding. Look it up here on this site, they have wonderful articles on how to help your bird.
Check for discharge from the nose, gasping, foul breath. She may have a respiratory infection or a sour crop. Check to see if her crop is empty, its the weird little bag in front of their breast. If she isnt eating much but her crop is full, it may be impacted.
There are so many resources here that can help.
https://www.backyardchickens.com/f/10/emergencies-diseases-injuries-and-cures
This is a good area to browse through, it might help you find symptoms you didnt know to look for.
Read read read before treating. Make sure of what you have before you start pumping medications into her.
I hope she will be OK!! its good her "Mom" is so watchful and attentive!