The social dynamics of your flock is bound tightly, as they've been raised together, from the brooder. This reality makes introducing an "outsider" an "intruder" extremely difficult. BTW, did you keep the new bird apart for a few weeks to check her? Just sayin'.
Additionally, chickens sense, somehow, when one bird is a bit "off". If she was pecked on by a previous flock, she's one of those birds that gets pecked on. If there's a weak link in the flock, it gets beat up pretty good. Chickens tend to do this. It's a tough world they create and maintain. The weak, sick and lame are not coddled, but are usually tormented. When a bird is under the weather or has an injury, for example, they are pretty expert at hiding that fact from the others in the flock. Just the way it is.