Food, Glorious Food! Oh how they adore it!
Sorry, I couldn't help myself - LOL! Seriously, I was having the same problem with my 10 week olds that had just moved into their very spacious and beautiful coop - only they didn't go in at night. An older friend told me to condition them to a call like 'hey, chick, chick, chick" while rattling a can with scratch. And I knew they loved scrambled eggs (kind of cannibalistic but what can you do...) so I started calling them and placing pieces of egg on the ladder into the coop. Then, because the coop is large, I got inside and enticed them in with scrambled egg from the inside. It took two nights and had a few "outliers" that had to be picked up and put in but now - a week later - they come running when I call regardless of where they are. I also think putting them to bed too early is also a problem as they aren't really ready to settle down. So now I go out at dusk, call them into the coop and give them a treat. Works like a charm.