Sometimes there will be one who just does not thrive as well as others, even the same breed, though they get the same feed and care. There is at least one strain of coccidiosis that can be chronic, and necrotic enteritis is another possible illness that can occur after coccidiosis. It is treated with common antibiotics, such as Tylan, amoxicillin (Fish Mox online,) penicillin, metronnidazole (Fish Zole,) tetracyclines and others. A local vet could examine some droppings for enteritis with a gram stain or stool culture. That may or may not be the problem. Chickens who have been exposed to Mareks or lymphoid leukosis can also be weak and affected by common illnesses due to low immunity. Feeding her separartely in a dog crate daily, then letting her out, may help if she feels threatened by others.
A lot of people recommend fermented feed here, making it sound like some much improved way of feeding. I believed a lot of the hype (just like ACV) when I first started with chickens, so started using it. After seven months, I stopped. There was no difference in my chickens, but I couldn’t tolerate the mess of spilled food killing the grass. It is great for a weed killer though. In summer, spoilage can be easy, while in winter I had to keep 5 gallon buckets of feed fermenting, since it freezes outside. My chickens got fat, had more runny poos because of all the water they were getting in the feed, and it was a lot of work mixing a new batch and keeping the thing going. I had 70 chickens at the time, mind you, so it might be easier with the smaller flock I have today.
Lab work for a chicken is expensive, but I would get some droppings check. She also may have a reproductive disorder, less common in pullets, but it does happen if you read the emergency forum here. I hope that you can figure it out, but if she continues to go downhill, I would get a necropsy through your state vet to find out what was wrong.