Sometimes when chickens are sick, and also during colder weather, they won't drink enough of the medicated water. If she isn't drinking well, I would get some Tylan 50 injectable from the cattle medicines t the feed store, a few syringes and needles, and give her some orally each day for 5...
Sometimes it is hard to diagnose chicken problems if everything looks normal on the outside. Check to make sure she isn't going broody, or wanting to hatch eggs. They will pull out their feathers on the chest and belly to cover eggs next to their skin. They stay in nest boxes, and will sit on...
Welcome to BYC. It really could be a lot of things, but I would start off with worming her with either Valbazen or SafeGuard Liquid Goat Wormer. Look over her skin especially under the vent and on the neck for signs of lice or mites, or their eggs. Check her (before the worming,) for a stuck...