It's not easy to get ideas from so little information, so I'm going to take a wild guess. Then I'll leave it up to you to investigate.
From her age, I'm guessing she's among the youngest of the flock. There is likely a hen just a bit older, maybe eighteen months, that is bullying her at roosting time, maybe preventing her from finding a spot on the perch or even from going into the coop.
Youngsters are intimidated by older chickens, and those chickens who were youngsters not too long ago, are now in a perfect position to engage in the delightfully entertaining (to them) game of chasing the younger ones. If a youngster is a bit on the timid side, she may give up rather than subject herself to conflict.
You will need to watch what your chickens do as they go into the coop to roost. If you find the bully, then you can take steps to intervene.