Hen will not go into coop at night

Hello
We have a small flock of 7 hens and 3 roosters. The hens stay in an Earthbag dome (9' diameter) with an auto door that leads into a 10'x20' all enclosed run. The roosters stay next to them in an outside 30'+ Round pen with lots of trees.
One of our hens (Buff Brahma 6 months old) will not go into the coop at night and stays out in the run on a perch that faces the roosters. Every Eve I go out and put her in the coop through the auto chicken door, then go inside and place her on a perch. Once I do this she stays there at least until the door closes.
Since the coop is round we have one spiral set of perches in the center of the dome. The 3 oldest hens take the top perch and the 3 youngest take the next one down. The hen that does not go in is right about in the middle of them all in reference to age. When I put her inside, I place her on the third perch down and move one of the younger ones down with her. (3 on top perch and 2 on each of the next 2 perches.) other than some chatter I believe they stay this way through the night. Each perch is long enough for 3 birds.
During the day, the hen who doesn't go in at night appears to get along with everyone else. She gets right in there with everyone else for treats but appears to hang more with the younger ones or by herself. I've seen her peck at the younger ones but gives the older ones space when needed so she is probably in the middle for the pecking order.
We live off grid and the coop is not near the house so getting her to go in at night is important especially since winter is around the corner.

Any suggestions? I'm thinking she is odd hen out but not sure what to do.

Thanks!

 

Thank you for the feedback. Although we do live in AZ it is up in the northern part so nights are getting cool.