Have these ever gone into the coop to sleep? Is sleeping outside new behavior or have they always slept out there? I often have to teach mine to sleep inside by locking them in the coop after dark until they get the message to sleep inside. I wait until it is dark and they are easy to catch, then toss them in onto the coop floor and lock the door
It sounds like you have had chickens previously that did sleep in the coop. If that is the case it eliminates some possible reasons they don't want to go inside at night. There may be some clues in your history that could help.
How big, in feet, is that coop that will comfortably house 18? Several times people have said things like that and it turns out that space is the problem.
If you can, a photo inside and out can sometimes help a lot. You never know what someone might see as a possible problem or what possible causes a photo can eliminate. As Rosemary said there are a lot of potential problems. The more we can eliminate the better we can guess as to what might be happening.