Those birds may be low on the totem pole, and their feathers are being pecked and possibly eaten by other, higher-ranking birds. Sometimes it is said that this is evidence of a protein deficiency. You may want to up protein a bit, and make sure they have enough space in their coop and run, as well as make sure they have things to amuse themselves, such as a flock block, hanging cabbage, bale of alfalfa or what-not to pick apart, roosts, branches, etc. If that doesn't take care of it, you may want to try pinless peepers, to see if you can break the cycle of picking. I was led to believe that it was not a great thing to do, but have used it on my birds without problem. They are regrowing most of their feathers now.