At this point, I'd try it for a day. If that doesn't work, try 2 days, etc..
I've had that with birds a bit older and it doesn't take long to reinforce them.
Yes, lock them in for longer.
Is there a ramp?
Young chickens aren't as inclined to roost or use a ramp so even though they use the ramp during the day, they're quite content to plop down in the run to sleep.
Does the building have big ventilation?