New chicken keeper with hen butt problem

I recently started caring for my neighbor/landlord’s chickens for him. One of the hens has had persistent butt problems. Poop seems to continually get stuck to her, and she has feather loss and skin irritation. She is acting normally. There are 8 chickens total, and none of the other chickens have a dirty butt.

I’ve added an herb/chili mix to their feed. I’ve also checked for mites/lice and haven’t seen any signs, but possibly I’m missing it? I’m not sure how to help her.

The picture is from today, about 24 hours after she had a butt bath.

Any advice or tips are welcome. TIA!