She looks to be, well, old. Given that she has been ill, and her age, her immune system is probably not up to par, and this partial molt is all that she's going to get until the next fall molt.
Like the others have suggested, additional protein may help. I would also give her Poultry Nutri-drench which will really boost her nutritional intake and may even help with feather re-growth, but I wouldn't hold my breath.
I had a very old hen (age seven, pictured above) who began molt in the fall with the others, but her feathers only grew back partially, and they grew in resembling a frizzle, which she wasn't. The winters here can be brutal, so I rigged a heat lamp in the run for her to warm up under. At night she was able to sandwich herself between two other hens so she stayed warm when she slept.
She developed a series of infections and finally went downhill to the extent I knew she would never recover. I decided to euthanize her. She never made it to her next molt so I will never know if her feathers would have grown back. I have my doubts.
Edited by azygous - 1/4/16 at 6:10pm