One chicken staying outside at night

[spymeg]

For the past week, we've had one chicken who won't go inside. Everything is clean, no bugs (that I can see), no predators and she isn't the lowest in the order, so I'm not sure what is happening. Maybe she just likes the warm summer nights? We have an electric door and at first, I was going out to open it for her and had to make her go inside. She has no interest in being in there at all. They are totally secure in their run and it's protected by an electric fence, but I still worry about her being out of the coop all night.
Any ideas?

 

[aart]

How big is your coop, in feet by feet?
Pics, inside and out, would help here.

How many birds total?
How old are they, in weeks or months?