Chickens missing feathers for months

[ice2fire911]

I have a couple of chickens out of 19 that are missing feathers on there back by their tail and on their underside. They aren’t growing back and it’s been all summer. I’m also only getting 11 layers out of 19...I’m feeding Purina layer for chickens. We also have no roosters only 19 hens

 

[Wyorp Rock]

I have a couple of chickens out of 19 that are missing feathers on there back by their tail and on their underside. They aren’t growing back and it’s been all summer. I’m also only getting 11 layers out of 19...I’m feeding Purina layer for chickens. We also have no roosters only 19 hens

Can you post some photos please?
What is your housing like, sq ft of coop/run?
How old are they?
What type of treats do you feed and how much?

Have you checked them over for lice/mites?