What's the protein content on the layer? How much food scraps? Try just feeding the 18% feed, and add in some animal based protein like cottage cheese, canned fish, or scrambled eggs.
What are you feeding them? How big is your set up. Feather picking is generally from a protein deficiency, or boredom. I would separate her back out. You may need to keep her separately for a few months to break the habit, but first you need to find out why she's doing it.