Welcome! I agree with sourland. If you watch them, there might be an instigator, and that's the bird to isolate away from view of the flock, for a week or two. She will fall down the pecking order when she returns. If things are wonderful without her, move her on!
Having space, places to be out of sight, and a very good diet, all matter here too.
What breeds do you have? Some are not as nice as others...
Mary