I imagine they need reinforcement as to the place you would prefer they sleep. I would confine them to their coop for about a week so they HAVE TO sleep on the roosts. That worked for me with my first batch of babies. They stayed inside for a week and then I let them out into the run and they always returned to the coop at dusk.
I agree with dozengirls confine them in the coop for awhile 5 days or so if that doesn't work then herd them in every night and shut the pop door till morning they will get the idea sometime. Really the jail time should do it though.
My silly girls won't go in at night either. Today it was raining a lot and I noticed that they were all inside. But come night time I had to drag them all inside, AGAIN! The weather here has been too hot to lock them inside but now that it's getting cooler I am thinking that the coop prison is a good idea. They won't like it since they have been allowed to hang out in the run all day. They really like it in the run but I'm really getting tired of having to drag them in at almost dark.
I have the same problem. Mine sleep in the bushes next to the coop. When I first got the chickens they slept in the coop all the time. But for the last month or so they sleep out side. I tried confining them for three days but it really didn't help. The only temporary improvement is that one of them sleeps on the roof of the coop. She now has the nickname Snoopy.
I made my coop from an old dog house. Here's a link to it. I'm afraid it may be two small for my six Easter Eggers. http://chickensoncamera.com/2010/04/custom-chicken-coops-my-story/
Any thoughts on what might help get them back inside? I've had people suggest that as it get's colder at night they will move back inside.
We took someones suggestion of putting a light on around dusk (to help them see to get to the roosts) anyway, and also hearded them into the coop, only took a few days and they always go in nowwithout they light.
Is there a particular reason you need them to go into the coop? If your run is secure, and they want to be outside, why not let them? I would, however, make sure there is not a reason they are avoiding the coop (like a trapped predator, or no ventilation). They should want some shelter some time, but if they prefer to go au natural, I'd let them.