Look at the feathers that they are pulling, you might see that the feather has a little pink inside, new feathers are called "Blood feathers" because of that. Just removing the 2 that are getting picked for a little while until the feathers are done growing and not fresh might be all you need to...
Sounds like the lower-ranking birds are the ones getting plucked. They will let the boss birds pick them to death, and never fight back. If you see blood you will know they are serious. Then the only fix is to remove the bloody bird until it heals, coat it with blue-kote and decide if the low...
Birds do not show their pain. Are the ones getting picked on acting normal otherwise? Eating, moving ok? How old are they? What breed - some breeds are more prone to feather eating, like sex-links. Feathers are protien, there are layer feeds that are 17% instead of 16% protien that might help.