Rhode Island Red Tail Down

[Sullnily]

I have a Rhode Island Red (Ann Margaret) with her tail down, it has been weeks. She is still laying. I have added electrolytes and B12 to the water. The only other symptom is her comb is pinkish and a little crusty. She is one of 5 chickens and so far all of the others look very healthy.
One suggestion was separation and dosing with Corid. Any other suggestions?
Thanks, Sullinily