The light on a timer is an interesting idea. Earlier I l posted my girls had started roosting outside. Now three are still outside and two have moved in. I do have an LED on for 15 minutes just before dusk. But I confused the whole issue by building a new "Taj Mahen" coop. It has a higher roost (3 1/2'). Small chicken door left open 24-hrs, gable vents and roofline ventilation. No change in bedding nor feed. Moved them less than 10 feet from the old spot. No eggs for over 5 weeks. An earlier poster mentioned chickens don't like change, and BOY, is that true! Here's a pic of my coop interior.I agree that your roosts look too low. Chickens naturally go for the highest place in the coop to roost. I occasionally have hens go to the fir trees if the coop door is closed for so random reason. You could try a light bulb on a timer (LED so not to add any heat), and time it to be on during dusk and then turn off after you close the coop door. This could attract them to the light and then they would settle down in the coop, on the roosts, and then close the door. When the chickens go to the trees I say they are going back to the wild...but the wild will get them in the end. A sleeping chicken is a sitting duck. lol
My roosts are as high as they can be with a standing rooster not hitting his comb.