I have 4, 13 week old chicks (2 dominiques and 2 barred rocks). 3 are hens, 1 is a roo (a Dominique that I'm trying to rehome). I started letting them out to free range with the rest of my flock at about 9-10 weeks. At first, they loved it and wanted to be out all the time. The past week or two they just flat out REFUSE to leave the coop. It's SO weird. Everyone else is outside and they just stay inside. One of the hens has started going out and has integrated herself in with the big girls, but the remaining two hens and roo stay inside all the time. Yesterday it was so hot and humid I literally had to pick them up one by one and carry them outside and bring them to the rest of the flock. Once they were out, they were fine. Has anyone else ever encountered this? It just seems so odd to me. They have access to free range and would rather sit inside a hot coop all day? WEIRD?!?!
They're individuals. Is there any bullying by the older birds? I usually open the door to the brooder house when chicks are about 3 weeks old after a couple days they all go out. But one flock of chicks I had refused to go out like they were afraid of everything. They were about 12 weeks before they started to venture out. Like you, I chased them out once and when I was gone they ran right back in.
I have 3 adults (the odd balls of my group) that don't leave the coop all day, they come out just before bed for a bit but that's it. I think they're just not fans of their flock mates. But i bet your little guys just got spooked they'll come around.