I had some that were afraid of the dark when younger. Inside the coop was darker than outside, so they would be afraid to go in I used the treat trick, but also had a light above their pop door . We toyed with them one night - turned light on, they ran in. Turned light off, they ran out. On, and in, off and out. ( Yes, we are mean LOL but I was actually experimenting to prove my theory about them being afraid of the dark.
After that, for new chicks,or when changing coops, I would shine a flashlight at the door from the inside for a few days until they were smart enough to go in before it got scary-dark in the coop
After that, for new chicks,or when changing coops, I would shine a flashlight at the door from the inside for a few days until they were smart enough to go in before it got scary-dark in the coop