How much light do you have outside? Do security lights or street lights keep it lit? The signal for them to go to bed is that it gets dark. How much natural light is there in the coop? They need to be able to see to go to bed, they can't see to move around if it is too dark. Some people have found that inside lights help but many of us have enough natural light from windows or ventilation that lights are unnecessary. It is possible that by the time they get the message it is time to go to bed inside the coop is already too dark. You mention that three sometimes go in on their own but it doesn't sound consistent.
I find that my brooder raised chicks usually don't start roosting until they are 10 to 12 weeks old. I've had a few start as early as 5 weeks but older is more normal for mine. Until mine start roosting they like to sleep in a group in a low spot. With my elevated coop that is usually on the ground under the pop door. With my ground level coop that is generally inside the coop but on the floor. I do keep mine locked in the coop section only for a week or so before I let them out to the run. With the elevated coop keeping them in for a week doesn't seem to matter, in my ground floor coop is does.
How I manage mine sounds like what you have been doing. Every night after dark I lock them in the coop section, just putting them on the coop floor. Sometimes they catch on pretty quickly. One time it only took one night and all 19 got the message. A couple of times it has taken three weeks for the last few to catch on. Usually a few start going in on their own within a couple of days and by a week they all catch on. Each group is different.
One thing that sounds different from mine is that once a chick starts going in they are consistent about it every night. It doesn't sound like yours are. It makes me wonder if there might be a light problem, say on a cloudy night versus a clear night?
