So these are young birds that are still building habits and routine, including going inside the coop. And this is their first fall, when the days are getting shorter and gloomier. If there are no windows in the coop, and the door is closed, then it must get pretty dark in there, especially on a cloudy day. Chickens have very poor low light vision. If the coop is significantly darker inside, than it is outside, they may be afraid to go in because they can't see. This is only going to get more problematic as we get further into winter, and the days get even shorter and gloomier. I would recommend cutting a nice, big window or two into the coop to give them natural light inside, and to even out the light levels inside and outside. You can use a discarded house window to stick in there, or a sheet of glass or plexiglass. Or you can buy a new window, if you can afford it.