There's no reason to perform all that labor. You don't need to teach chickens to roost. Like you said, something just kicks in. Trying to force them to roost by putting them up on the roosts before that is a waste of your time. No need to be persistent, but rather, just be patient.
Some of us are impatient.

(I don't have the right to complain, my chicks started roosting at just past 4 weeks old.)