Chronic Diarrhea in Hen

[Heavenlyhenspdx]

I have a 3 year old Easter egger who has always had runny stool. As a chick she was healthy and clean, but she had runny stool. She’s still acting fine and laying fine, but her feathers around her vent are caked with poo and I can’t get it out. She’s been to the vet and did a round of antibiotics but that hasn’t changed anything. The other hens have normal stool and clean bottoms. What should I do?