In addition to the vitamins try feeding her high protein foods - shredded cheese; cooked eggs; dry cat food wetted down & mashed up. Ruth had same symptoms and I gave vitamins and hand fed protein foods till she recovered so not sure which worked, maybe a combination of all of the above. Take a look at Ruth today by clicking on my home page. She's beautiful but notice how sick she looked at 10 weeks old.
I'm probably the only person on here who does not believe that the commercial feeds are everything they are cracked up to be. Chickens are omnivores and need the "living" foods like grass, veggies, fruits, bugs and not the "processed" foods like commercial feeds that were designed to keep a chicken alive long enough to kill it and cook it. Mine free range all day and get lots of extra foods. Never had a problem with any of them since I stopped feeding chicken feed exclusively. Now it's in their feeders and it's the last thing they eat.