Feather loss

[Bluebirdhens]

Hi, one of my one year old Easter egger hens has been developing and increasingly large bald spot on her back. I thought it originally started from a very large rooster we had, but it’s just been the hens for the past six months and the spot continues to grow. I don’t see any visible insects and the other three hens are unaffected. Am I missing something that more knowledgeable chicken people see? Her behavior/eating/laying are all still normal. Thanks for your help!

 

[Wyorp Rock]

One (or more) of the hens are obliging her of her feathers.
She will regrow feathers when she molts.