Might be Vent Gleet. I have an older pullet who could not keep a clean butt for the life of her, once she got her adult feathers. I ended up having to bath her bum regularly and trim all fluff and feathers in the area very short to keep them from getting soiled and amassing poop. She didn't have worms, and was eating and drinking normally. Essentially, vent gleet is a bird yeast infection and can cause diarrhea poopy butt if other causes have been ruled out.