I've had that problem before too. They were sleeping on the roost in the tractor, then one or two would decide to sleep on top of the tractor. I'd wait until it was dark, then put them in the tractor. If they could see to get on the roost, fine. If it was too dark for them to get up there, I considered that their problem, not mine. They did learn to sleep on the roost after a few nights, but a week or two later, we'd go through it again. Although I had plenty of roost length, I think my roosts were a little too close together which may have helped cause them wanting to sleep outside.
The light might help. Obviously it has worked for others so it probably worth a try. When mine would sleep on top of the tractor, they still had plenty of light to get to the roosts. They just preferred to sleep on top.
Good luck! It can be frustrating.