My guess would be either lice/mites, or you have a feather picker. Cleanout the coop and nest boxes and treat the coop and your chickens for lice. Give them a tub with sand and wood ash for dust bathes. If that doesn't stop the feather loss, you might try giving them a flock raiser or all flock feed, they contain more protein than layer pellets. Feather picking can be caused by not enough protein or overcrowding. First make sure that your chickens have more than enough room. If that's not the problem, upping their protein intake should resolve the feather picking and help them grow their feathers back. They might not look normal again until after the next molt.