We have had the same exact problem. It started last spring. We had 1 hen that was missing the ends of a bunch of feathers on her back near her tail. That issue eventually turned into the feathers missing altogether. We tried Vicks, peck-no-more, blue kote. Nothing really worked. We suspected that it was our dominant hen, but couldn't catch her in the act. I was about to order pinless peepers for her, but while we were away for the weekend, the suspected pecker and another of our hens vanished. (MDH insisted on our friend letting the hens out for the day while we were away) After that hen went missing the picked on hen began growing her feathers back. It was an AHAAA moment. "So, it WAS the dominant hen!" "Well, that problem is solved." NOT...now another of our hens is missing the tips of her feathers on her back near her tail. And this evening I saw a bare patch on her back where they are actually being pecked out. I haven't seen anyone display aggresive behavior to another hen since the suspected bully went missing. So...now what? Back to square one. It must be happening while they are in the coop/pen before they are being let out.
I believe it has something to do with boredom this time. We found out that we had a fox problem. Two weeks after we came back from our weekend away and found our two hens missing, I was returning home when I saw one of our hens being killed in the front yard by a very large and healthy fox. My son shot it, but didn't kill it as we saw it run away. We have been very cautious about letting the hens out unsupervised, so they aren't getting out for as much time as they were used to. I am the one that is worried about them getting picked off by another fox when there isn't someone home to defend them. (I did locate TWO dens in the hedgerow on the farside of our property!) I think that the best buddy of our now missing bully, has learned the picking behavior and is trying it out on her buddy. Unless it could be the hen that was being pecked before trying it out on this other hen. I never did catch the behavior the first time. I was going to try the peepers on that hen and was hoping that the feather picking would stop so that I didn't have to try them on each and every hen until the picking stopped. I'm wondering how much a camera system would cost to try to catch someone doing the picking?