Hello there and welcome to BYC!
As Sunflour has suggested, make sure they have enough room. Cramped chickens pick on each other. Give them things to do...free ranging, ladders, mirrors, seed pecking blocks, hanging cabbages or apples.
Make sure the bird that is being picked on is healthy. Other flock members know when a bird is sick and they try and run them off to protect the flock. So give this bird a once over to make sure she has no health issues. If they need to be wormed, do so.
Also, make sure you have enough nest boxes. Sometimes 1 to 4 hens is not enough. Sometimes 1 to 3 or even 1 for ever 2 is better. Depends on your flock.
The lowest in the order can get picked on a lot. Make sure you have several feed and water stations out so she can get enough to eat and drink. They may be starving her out.
If all else fails, you can use pinless peepers on the meanies. (generally there is one that starts it and is the most aggressive) or you can jail the meanies for a while in a wire dog crate. Jail can do wonders for the attitude of a bird. Isolation for a week is lonely!
And always cover up all wounds with blu-kote. Birds can kill birds with wounds when they see skin and blood.
Good luck and welcome to our flock!
