It sounds to me like she just isn't getting enough to eat for whatever reason (small poops/goes crazy for food and scratch.) Are you sure she isn't being bullied and is getting enough to eat?
I had a hen that was losing weight so I paid more attention to them and even though she was up there in the pecking order, her slightly timid personality let a couple birds keep pushing her off food. I did multiple feed/water in different areas but every night her crop was smaller than the others. I had to just feed her extra at the end of the day which did help.
I'd probably pay extra attention to them when you feed or give them scratch. Watch the body language and see if your others aren't subtly pressuring her in some way. Also, check their crops at night after they roost to make sure she's getting enough. You might just have to make her some mash and let her eat seperately for a few minutes every night.