When I first started reading this thread I thought, yeah this sounds typical. Even more so when I read it was an elevated coop. I have a lot more trouble with this if the coop is elevated than a ground level coop. I think that's because until they start sleeping on the roosts they like to sleep in a group on the low spot. The low spot for me is under the pop door. My solution is to consistently lock them in the coop at night until they learn to put themselves to bed up there. My broods are typically around 20 chicks. I had one group put themselves to bed up there after only one night of putting them to bed up there. A couple of times it took three weeks for the last few to learn to go in. Each group is different but it's not unusual for them to all learn to go in after a week to 10 days.
Then I read this.
They don't go to bed until it is getting dark. The signal to them to go to bed is that it is getting dark. I wait until they have put themselves to bed under the pop door to start putting them inside. They are a lot easier to catch if it is dark enough that they have already settled down for the night, though the last ones seem to wake up.
I think you taught them where to sleep a long time ago but haven't been giving them a chance to do it themselves.

This. Exactly.
I find it nearly impossible to get my birds herded into the coop before it's full dusk.