Hen being picked on

I noticed about 4 days ago, our only buff Orpington hen was missing a lot of feathers on her backside. She seemed to be acting normal so I picked her up to look and noticed a little yellow looking small worm on the bare spot. Picked her up the next day and never saw it again. I continue to keep an eye on her and she is getting very shy acting, but a few days ago she would still walk around the yard with the rest of the 10 hens and a rooster. Yesterday and today she will not leave the run, she doesn't sleep on the roost like she usually does. Instead she sleeps on the ground in a corner. I noticed the rooster and one other hen were picking on her the most. So I separated them this morning into a safe pen that is close to our big coop, I am going to watch for more dominant hens and separate them for a few days too if she continues to be picked on. I haven't noticed her eating or drinking and I try to dip her beak in water or food and it doesn't phase her. Not sure where to go from here so any help is appreciated!

She is doing worse today it seems like, won't even leave the coop

It could possibly be that she has some kind of sickness and that has caused the attacks - chickens try to drive sick flock members out of the group. You may wish to post any symptoms in as much detail as possible and post on the "Emergencies, diseases, injuries and cures" forum for advice (as well as leaving this one open).

All the best
CT