I would increase her protein--try giving high-protein treats like mealworms, wet cat food, scrambled eggs, etc. You could also switch to a higher protien feed, like game bird grower, and see if that makes a difference. I would definitely check for mites/lice, too.
To check for parasites, look under the hackle feathers and wings, near the oil gland, and near the vent. Mites will appear as small black moving specks, and lice will appear as moving yellow blobs. If one bird has mites/lice, chances are all of your birds have them.To treat mites/lice, dust with poultry dust or 5% SEvin dust, or spray the bird with another mite/lice spray. Clean the bedding out of the coop, spray the walls and floor with vinegar or Sevin liquid, and put in fresh bedding.
It could be that she is just getting old. I've heard that production hens (Golden Comets, Red Stars, etc.) tend to have difficult, sometimes incomplete molts, especially as they get older.