If they've never gone in, they may not realize it's their house. You may want to try a lock in to get them to associate the roosts inside with their nighttime routine (search for the term; lots of threads).
If they've gone in previously, but have stopped - then you might have the problem I had (mine started roosting on the back porch). Thanks to folks here, we realized it was either a ventilation or lighting issue. We set up a rope light on a timer, and also installed a dual-fan in one of the windows - it circulates the air and makes it cooler in the coop. I think my girls just preferred feeling the breeze in their feathers, so we supplied that in the coop, and they're all happily piled up inside.
Good luck!