Have you checked her crop? Are her droppings normal? Diarrhea?
Foods my birds love as a treat and daily are moistened dry dog food, hard boiled eggs, dry meal worms; any treat she ike and will eat.
May be put vitamins in her water. The below might be helpful from farmfowl.com.
Here are some of the benefits of Apple Cider Vinegar for chickens
Chickens have one of the most efficient digestive systems in the animal kingdom. One part of this is called the crop and this is the first place the food goes after the chicken has eaten it. The food is broken down here and gradually passes to the main stomach. ACV helps to reduce the pH level in the crop which helps to combat any ingested microbe and bad bacteria.
Apple Cider Vinegar can help reduce internal worms in chickens. It helps to make the chickens gut not a nice place for worms to live. It makes the chickens gut a less hospital place for worms to live. It won’t completely eradicate them, so regular worming is still very important.
Apple cider vinegar can act like a mild antibiotic. Antibiotics kill infectious bacteria that can cause disease in chickens. We have used it ourselves if a chicken appeared to look a little low and I have recommend it to others.
In addition to these benefits, Apple Cider Vinegar is full of vitamins, minerals and trace elements that are beneficial to your chickens.
Dilution rate for Apple cider vinegar for chickens
I dilute ACV at a rate of 5ml or a tea spoon per litre of water and give it to the chickens for 3 to 5 days each month. I found that giving it more often that this can reduce the usefulness of the chickens. This is just my experience and what has worked well for me over the years.
Never use ACV in a metal drinker as the acid reacts with the metal. It should only be used in a plastic drinker.