I had this problem with my leghorn. I tried increasing protein, vitamin supplements, letting them free-range for part of the day, etc., but she still kept plucking.
She is a good layer, and rather than cull her, we went the pinless peepers route. It worked.
If you decide to try the pinless peepers, my advice would be to put them on in the evening. I tried doing this once in the daytime, and the bird freaked out and the other birds all attacked the peepers.
The second time, we went out at dusk. I wrapped the bird in a towel, and DH used a headlamp and needle-nose type pliers to apply them, and then we set her back on the roost. By morning, she seemed to be comfortable with them.
It will restrict the bird's vision somewhat, which concerned me when they are free-ranging, but I was going to have to get rid of her otherwise. She has worn them for a year now, and is doing fine.
I got mine here:
http://www.eggcartons.com/Pinless-Peepers/productinfo/PINLESS-P/
Good luck!
