Check her thoroughly (at night) for parasites. Look particularly carefully around the back of her neck and on the vent. Look on the feather shafts for eggs as well, or breaks in the skin.. Feather lice and mites will cause this. Using a flashlight and putting the bird over a light colored sheet as you check for parasites helps. Mites will go off the bird during the day in some species and you won't find them - just symptoms. So that's why you check at night. If you find any parasites at all, you must treat as if it's an infestation (rather than just using prevention methods). That usually entails using a poultry powder to dust the birds, and then treat the premises thoroughly including white-washing the wood of the coop using water mixed with poultry dust to get in the cracks and crevices.
You must retreat at least once in seven days.
A lack of proper protein or nutrients will also cause this. As will feather picking from the other birds.
What is her diet exactly? Her age? Any grubbiness or sticking droppings near her vent? Is she kept with other birds?
My chickens rarely molt in a head-to-tail manner. That seems to be more clear in forced molts. Mine just molt where they molt, so if you're seeing any pinfeathers coming in she might be molting.
