When my Light Brahma had that problem, I discovered she was picking her own neck feathers out as soon as the pin feathers came in. I cut the foot off an old sock and made her a neck gator. But she quickly removed it.
I sewed two elastic loops onto the bottom and slipped it over her head and the elastic loops under her wing pits. That did the trick. The feathers grew back and I removed the gator and she's not gone back to the annoying habit.
If I see the problem coming back, I'll make a neck gator that velcros in the back since slipping it over the head is hard to do.
I also had some success with putting Vick's mixed with grape-flavored kool-aid on the picked area. It stains, though, and is very, very greasy.
Feeding tuna and cat food to increase the protein intake also helped discourage the picking.