I have 3 Isa browns. 1 Rhode island red, 2 ducks, 1 sex link Rhode island/ barred rock mix, 5 barred rock. I have 2 chickens out of the 10 total that have had no feathers on there behind, for probably close to 8 months, at first I thought molting. Which has only happened 2x in older birds so far, but they just never grew back, I used to have some more aggressive chickens and I think it started because they were being pecked at. the skin above their tails has been bright red for some time so naturally I though it got infected a skin fungal or bacterial infection, no wounds that I ever remember seeing, and bright red for at least 5 months, I have tried, acv orally, banxx I think it's called, an antifungal/antiseptic, gentle soap and water, blue kote x 2 daily, I've added electrolytes to water have good feed and calcium ava, and nothing works. Their tail fathers are kind of scraggly they eat well, poops fine, produce eggs and they dont seem bothered, they keep up with all the other chickens drink water. The sex link has started to get some bald patch on its behind now too which is new but no redness. I have also done de but stopped recently because it's 116 out here still and I have to wet down the ground which makes it almost useless. I have checked them for mites and have never seen obvious signs and nothing directly at vent. Nothing wrong with eggs either. Aside from completely cleaning out run, using de on birds and run, what else can I do