I had the same problem not long after I put 50 BW chicks (6 wks. old) in a 27 sq. ft. pen. It progressively got worse as time went on. They had a high protein feed (Nutrena Starter) in front of them at all times. It was the crowded pen, and I knew it. Several of them looked pitiful from the lack of head/neck/tail feathers. I built a 60 sq. ft. pen and put them in it, and the pecking problem fixed itself immediately. A fourth of the pen is crammed full of pine limbs to go hide in. All have full feathers now, where several had none on the head and neck. All had no tail feathers, and now they all have tail feathers. Don't get me wrong, they still chase each other from time to time (mainly the roos), and try to peck, but there's enough running room and hiding space now that the chasers/peckers give up quickly.