Our coop is set 4 ft above the ground and the space underneath is incorporated into their large run. We have a ladder going up to their coop, and it took a couple weeks for them to use the ladder to get up into the coop, and that is after I put a battery operated light inside.
I had to go out there every night and pick them up to put them to bed. Even rain didn't make them go to roosty time. They could certainly use it to get down, but not up. I put some traction tread for stairs on the spaces in between the slats on the ladder, and it still took another week for them to go up. Now they do every night, even without the light on. The coop is like Ft. Knox, so it's safe. Just took awhile!