Observation is the key....my bet is you've got a particularly mean chicken--could be a hen or a roo, that is pulling the feathers out of the rumps of everyone. Are they penned up alot? Maybe a bit crowded?
My small flock had this problem, and so one day I just sat quietly and observed for a while....and it was a very nasty New Hampshire Red....she would come up behind each chicken, take a look at their backs, and WHAM! Rip out a feather or two. I got rid of her, and now everybody's feathers are back and they're all happy campers.
Watch and you will see....
Sharon