I had the same problem until I put a light in my coop- it's on a timer to go on half hour prior to dusk and a half hour after dark.
It also goes on half hour prior to dawn in the winter, when I've switched out the compact fluorescent for an incandescent to warm it up a bit (in addition to the ceramic heating element I use all winter).
This draws them in as the brightest spot in the yard at night, and they prefer to go into a lighter area than an area that's dark (who doesn't?).
You can get an automatic, solar LED porch light for about $50, and I know there are others that are light triggered, but I'm not sure where to find them, if you haven't power to your coop.