We've been battling with the feather pecking for 2 years now. Other local friends have the same problem, even with coops and runs whose dimensions far exceed the recommended..... (I think ours is under control now, knock on wood).
From my own experience, I think it starts from boredom and protein deficiency. Then once the habit gets started, it's really hard to break... and other hens pick up on it. We re-homed two of the worst offenders, but the others kept it up somewhat.
We tried everything: hot-pick, vaporub, blue-kote, etc... nothing worked.
it's definitely worse in the winter, when they are not out foraging.
here's what we do now, which i believe helps: every morning we bring a bowl of kitchen scraps, including vegetable peels, wilty lettuce, parsley stems, and, most importantly, meat and fat scraps. if I don';t have meat and fat scraps for a couple of days, I'll open a can of tuna for them. This morning treat gives them something to scratch around for (scattered in the run) plus extra protein.
Then, when I go out to check for eggs in the afternoon, I sprinkle a cup of black oil sunflower seeds in the run. Again, something to do, plus extra protein.
When the hay in the run starts getting matted and they've picked all the seeds out, I add another armful; usually once a week. Something fresh to pick through...
I really beleive this has helped.
Also- when we have an ailing hen, we always give AviaCharge 2000, which is a wonderful immune booster. It also contains lots of amino acids and micronutrients which hens aren't getting from insects in the wintertime, so we occasionally give it to the entire flock (in their food or water) to combat protein/nutrient difficiency.
Good luck. Try to nip it in the bud before it gets really out of control. My friend's entire flock got so bad that they NEVER had any tail feathers!
if you can figure out who the main culprit is, you might want to consider finding another home for her....
Best wishes
Stacey