It sounds like you have more than one breed in there?
I found, when mine were little chicks, that having more than one breed sometimes makes one breed a target.
However, i had success by keeping a really good eye on them, and whenever anyone is getting pecked, i use a blue-colored wound-kote spray on the wound, usually repeating the next day. This kept my babies safe and made so that i could put them back in with the flock
Also, as purpletree mentioned, too much heat will encourage this behavior. Describing their set-up, etc., might help us help you more.