Bare backs and bottoms

[Louavull chicken lady]

All 8 of my 19 month old RIR mix ladies lost most of their feathers on their backs and bottoms about 9-10 months ago. Their egg production did not slow over the winter, but has reduced by about 40% over the last 2 months. I have been told by some experienced backyard chicken parents that they are molting, but I am concerned it has been so long with no noticeable feather regrowth.
They are housed most of the day in a 6x8 coop with an addition 6x8 covered fenced run. I free range them most days for a few hours a day. I use sand in both the coop and run floor and pine shavings and straw in their nest boxes.
I have checked the ladies, and their coop and run, thoroughly for signs of mites and lice and cannot find any evidence of such. They have a dust bath box at their disposal with a mixture of soil and DME. I feed them Purina Layena and supplement with additional crushed oyster shells and dried meal worms. They are also spoiled with leftovers and garden goodies.
Pics are attached. I am looking for some advice on what I can do to make sure they are healthy.

 

[The Angry Hen]

It looks like they are just being terribly pecked on. Once I had five hens with that maybe worse, turns out the flock had a bully. Is there one hen or rooster that doesn't have that issue? They might be the trouble maker.

 

[Kiki]

Do you have a rooster.

DE will not get ride of lice and or mites.
I'd get some permethrin based spray and treat both the coop and the birds asap.

Limit all treats and switch them over to a grower feed for a few weeks, the higher portion will be good for them.

 

[Wyorp Rock]

6x8 coop with an addition 6x8 covered fenced run

Hi @Louavull chicken lady Welcome To BYC
Do you happen to have a rooster?

I agree with @The Angry Hen this looks like feather picking/plucking. This can be due to overcrowding, boredom and sometimes mites/lice. At 19months old, they may be getting ready to molt, but if they have been like this most of the year, then feather picking is your main problem.

Can you post some photos of your coop and run? Having more run space, if you can may help with some of the picking. Ideally run space should have at least 10sqft per bird, some do o.k with less, some need more. If you haven't done so add some perches to your run so they have a place to get away from each other, add something in the run for them to pick at - hang a cabbage, bundle of kale, etc. In the coop, if you have room - adding another roosting bar may help ease tensions as well. Free ranging for a few hours each day is good

Keep us posted.