She is young to be molting, at least a big molt, so do you think that the others are picking on her? They may be pecking her feathers out, and keeping her from coming out. Do you see lost feathers in the coop, which would be a sign of molting? I would watch them together, and make sure that there are at least 2 waterers and feeders sat far apart. You can apply some Nustock Cream, a bad tasting, antifungal safe cream that will mask red skin and promote healing. If you have a bully, then separate that chicken for a period of time.
