The condition you describe may be caused by an underlying problem unrelated to normal molt. I've had a few chickens go without feathers over large bald areas for up to a full year.
Currently, I have a one-year old hen who never molted when she was due for her first molt, and now her feathers are barely providing cover. Otherwise she's not acting as if anything is wrong. I'm waiting things out for a bit longer before I try anything.
It's been suggested to me in the past that you might be able to shock a chicken out of being "stuck in molt" by feeding lots of protein. You need to be cautious if you try this as high protein diet for chickens over long periods may result in other disorders such as poultry gout and it can put a strain on the kidneys and liver.
You might also buy a product called "Feather Fixer". It's a bit pricey, but is specially formulated to encourage feather growth.