I can't figure out the problem

[Shadowglare]

I've been taking care of my girls for months and the problems haven't changed. One of the Golden Sexlinks is losing feathers all around her vent (tail is fine) and moving the feathers before her wings are missing, it's more skin tone with a small patch of red. Then one of my Rhode Island Reds is losing feathers behind her ears and it does down half way by her neck, like they were broken, the area is more of a light skin tone.
This has been going on since February and I thought it was getting better, but it hasn't gotten better.
I've been using a spray for lice and mites, Food Grade DE, been cleaning up the coop everyday and getting the laying box spray and cleaned with new pine shavings and herbs, I even grab all my girls and go in the shop in the dark with a black light looking for anything that would make sense.
Let me know and I can take pictures if needed. All my other girls are good and I can't figure out what's going on.

 

[SueT]

Are they picking at each other‘s feathers? Do you have a rooster?

 

[sourland]

Sounds like some bird/birds are feather pecking - plucking.

 

[Shadowglare]

Are they picking at each other‘s feathers? Do you have a rooster?

I don't have a rooster, and they don't to peck at all when I'm around so they could be when I'm not around.