Hmm well I know some folks like having chicks decide for themselves when to start roosting, but I think in this case I'd start encouraging them to use the roost at night, just because chicks can occasionally hurt each other if they pile up at night. So instead of shutting the pop door at dusk, just let them go in as they please, then wait until it's very nearly dark and go back out. Pick them up off the floor and place them on the roost (you can also close the pop door at this time). Don't be surprised if they try jumping off or cry in confusion, because this is new to them. But they're old enough that they should start getting the hang of it in a few days.