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Mine prefer sleeping in a corner on the floor of the coop until they are around 10 to 12 weeks old, then they start to roost. Others have different results. They are all different.
Until they are ready to roost, they are looking for a "safe-looking" place to huddle together for the night. Evidently, under your elevated coop seems the safest place to them. They are creatures of habit. Once they get used to something, they can be hard to break that habit.
Have you tried leaving them locked in the coop for a week or so to get them in the habit of sleeping in the coop, then let them play in the run? It's all about them deciding that the coop is the safe place to stay, not just somewhere in the coop and run.
Another option is to keep doing what you are doing. They should get the message.
However, there might be another problem. How dark is your coop. Chickens don't see very well in the dark. Sometimes when something like this is happening, the coop gets too dark for them to see to go to bed before it gets dark enough outside for them to get the message it is bedtime. Lack of windows in the coop, position of the coop window, or outside lighting, such as a security light, might influence this. I'm not saying this is definitely your problem, but it is worth considering. You might want to consider adding a window to the coop or putting a small light on a timer to help them see to go to bed.
Once they get used to spending the night in the coop (I would wait until they are roosting in the coop) they will return to the coop at bedtime. Whe you start free ranging them, just let them out an hour or so before their bedtime so you can be with them. That's more for your benefit than theirs, but they may need some help the first time or two. I've never had them not want to return to the coop to roost, but I have had them trap themselves where they cannot. Remember, we do lovingly call them birdbrains. I have a 12' x 32' run with only one gate to it. I have had them get on the wrong side of the run and not realize there is a gate on the other side. They desperately want to get to the coop but the run is in their way. I've had to walk them around the run to the gate. Most get in OK to start with without my help, but there are often a few I have to help the first night or two.
