I would physically check my chickens once in a while for lice and mites, especially under the vent. I got a big surprise recently after a friend gave me a new young rooster. I always pick mine up and look for bugs, bumblefoot, vent gleet, and feel how skinny or fat they are about once a month.
Check her for lice and mites especially around the vent. Has she been wormed recently? They can act kind of puny when they are in molt. If her crop is squishy and balloon-like she may have sour crop. Here is some...