When mine were a bit younger they wouldn't go back in either. They would just gather together and start peeping loudly as if they didn't know what to do.
I decided to try some chicken psychology on them. First theory I had was that the inside of the coop was actually darker than the outside so they didn't want to go in this dark place. I put a light bulb with a timer on it that turned on at dusk and stayed on for a couple of hours. That seemed to work pretty well and they definitely started going in at night, whether because of the light or just chicken instinct kicking in. The problem as I saw it was that they were in a brightly lit room when the timer ran out and since they don't have watches were caught in sudden darkness without having properly roosted to go to sleep.
One evening they all went prior to dusk so I went ahead and shut the pop door and locked them in. I turned off the timer so they would get used to a slow fade to darkness. When I checked on them later they were all properly roosted and sleeping nicely.
Net night I stayed out there at dusk with, no timer light, to see what they would do. No joy. They were all staying outside. Then it struck me that they were all staying out with ME. As long as I was out there with them, they would stay outside. I left and went inside the house until it got really dark and when I went back out, voila, they were in. Unattended now they will go once it starts getting dark. If I do happen to be out there and they aren't going in I simply walk away for 10 minutes or so and when I come back, they're inside.
I am not by any means a 'chicken whisperer' and don't claim to be an expert, just telling you what worked for me.