Flock loosing feathers

[CoreyW04]

Hey guys, here’s an update! I separated the hen that was plucking feathers off of all the other chickens and I also was giving the chickens an added protein to their diet to help the feathers grow back.. I also was giving them apple cider vinegar to help with any mites( I’ve heard that helps with them). On Saturday I found one of my hens dead in the coop with no signs of an attack from predators or anything.. anyone have any insight what I can do to help my flock?
Thank you!