I have that same problem with my elevated grow-out coop. Even if they have been housed in it for over a week they try to sleep under the pop door. They don't have to use a ramp or steps (I use steps), they could fly up if they want to but they just don't want to. Since some of yours use the ramp I can't imagine there is anything wrong with your ramp.
One common problem is that the coop is so dark by the time they are ready for bed that they can't see to go in there. People have had success putting a light in there until after dusk. That's not why mine do that, my coop has plenty of natural light. Since some of your go in I doubt that is your problem.
What I do is go out after they have settled for the night and put them inside, locking them in. They are easier to catch if they have settled for the night. I typically have around 20 chicks in a brood. One time I had them all go in after one night of putting them in, a couple of times it took three weeks before the last few went in on their own when it got dark. What I typically see is that after a couple of nights a few start going in. Within a week they all are, but each group is unique, you can never tell how they will react.
Typically about half of mine use the steps to go up, half fly up. A few use the steps to go down but most fly down.