Of course, the LAST place you want her to be sleeping! Curious she chooses a place that if there were any light, would be the place with the LEAST light, yet she will sleep in the shavings if there is a light there. I think you need a Chicken Whisperer.
I have heard of the method espoused by vmdanielsen to get young birds to use the roost, put them on it and they will get used to it. Perhaps it will work for an older chicken as well. Does she get along with the other chickens so they won't pester her off the roost if she tries to join the slumber party? Will she sit on the roosts during the day? That might be a silly question though because I don't think any of my girls go up on the roost during the day. At least not if they are let out. And if the are still in the coop when we go to the barn, they will all be on the floor crowding the door before we could get close enough to see if they WERE on the roosts.
Bruce