Your chicks did a splendid job of weaning themselves off heat. Some don't do as well, and are like thirty-year old humans who still want to live with their parents. They're inside your house for weeks pumping dust and dander into the air.
As well as your chicks are adjusting to life in the big world, they may have confusion once they begin to go outside in the run during the day and it comes time to go into the coop when night comes. The reason is one that most people fail to notice. Simply, the coop, which represents safety, just doesn't look the same from outside in the run as it does from the inside. Chicks are loathe to go anywhere strange and unfamiliar,.
This is why a little night light installed inside the coop for the first week or so can make all the difference in the world to chicks who are learning where they''re supposed to sleep at night once they're out of the brooder.
If they still don't quite catch on, you might have to climb into the coop and coax them inside, but try to resist the impulse to grab them up and stuff them in. You don't need that stress, and neither do your chicks.