If it’s not getting bigger, I wouldn’t worry too too much about it. For now I’d just monitor her and make sure she’s not injuring it somehow or getting pecked at.
If the other chickens see blood, they will peck at it.
I think she could be rubbing her beak on something.
Could she be rubbing it on a feeder, or getting it caught on something?
Have you noticed it getting bigger at all?