Looking for suggestions for a hen that won't go into the coop

We have a small flock of six hens. We currently have an Easter Egger, a Olive Egger, two Red Sex Links and recently added a barred rock and an ameraucana. The Ameraucana is a pullet and is having a really hard time adjusting. We introduced both her and the barred rock slowly, over the course of about three weeks. The barred rock is fully integrated into the flock, but the Ameraucana is still refusing to go anywhere near the others, to the point where she won't go into the coop at night . We live in Ontario, Canada - and our winter's can get quite cold (down to -40 C last year) - I want to make sure she learns the coop is a good place to go in the cold, but am unsure what to do. At this point, I have to go out every night and physically put her in the coop (which she does not like at all ). Any suggestions? She's a beautiful bird and I would hate to lose her to the cold!