Feather pecking can be very severe...
If it is feather pecking the ends of some of the existing feathers will have a "v" shape at the end where some bits have been broken off. Like this:
One of mine was particuarly picked on, her back half being completely bald! :'( The bare skin on her back half was very red and dry.
I took her to a chicken vet. He said if the feathers were constantly pecked out they would give up trying to grow back after a while, hence the bald skin. He said the next moult would trigger her feathers to grow back, and it did. If not, then he could give her an injection (I think it was hormones) to trigger them to grow back.
The other reason some of mine have gone a bit thin feathered on their rears was if they were getting broody.