(2) of my adult chickens are raw and very red on their back near the tail. Can't see any mites, etc...but it is flaming red. I'm pretty sure they are NOT being plucked by another chicken. Stripped the coop and boxes. Used a poultry spray on the inside and dusted the nesting boxed beore re bedding. the spray actually gave some relief for a day or two but today they were bad again. I could try the powder...any suggestions?