You probably first want to ensure they are getting enough protein in their diets - I feed 22% protein feed and supplement with BOSS. (FYI, I had the same problem). I also put out flock blocks to help keep them interested. When I ruled out dietary issues I purchased pinless peepers from nationalband.com (the cheapest) and put peepers on the offenders. They are easy to put on, not permanent and do not negatively impact the hens other than they cannot focus forward to pluck out feathers. It has made a huge difference! I plan on taking them off in a few months to see if the behavior has stopped. Good luck!