How old is she? Did she have any vaccines? Can you clean off her vent with soap and water, and try to get a good look at her vent, just to make sure there is nothing out of the ordinary, such as maggots or a prolapse? I would give her vitamins and electrolytes in her water or give her 2 ml of Poultry NutriDrench orally each day—a couple of drops every so often. Offer her some chopped egg, tuna, or liver, in addition to her feed. Wet feed with a lot of water is usually accepted by a hungry chicken. Pictures of droppings are welcome.