Is there a window or any other light source in the coop part itself? If not and it is completely black in there they won't go in. Especially young chickens that, like any teenager, like to stay out late and run to the coop at the last shred of daylight. Perhaps put a nightlight in there until they figure it out? You could use one of those battery powered little things that you can stick anywhere like under cabinets or in closets etc.
