I suspect that she may be sick, possibly with a reproductive disorder or something else, possibly even Mareks, but that is way too soon to think about. There are many possible organ problems, intestinal problems (coccidiosis, enteritis, worms) that can cause illness or failure to thrive.
I would check for a stuck egg by inserting a finger one inch inside her vent. Normal would feel like soft tissue. Since her comb is not red, I kind of doubt that she is eggbound, but good to rule it out.
Check her crop to see if it feels empty and flat, partly full, hard, or puffy like a balloon. A crop disorder could prevent food from being digested and absorbed. If you can post a picture of her usual droppings it would be good. Green color can be from not eating, and white urates in a large amount can be from dehydration or kidney problems.
If you have a vet familiar with chickens it can be helpful. In case of coccidiosis, a fecal test might be a good thing to get, or give some Harkers Coxoid to treat for possible coccidiosis. Make sure that she can reach her food and water.