I have a very similar problem. Chickies prefer the tree in the back yard instead of their coop. We did lock them up for 3-4 days in the house when they were first moved outside and were not let out of the coop/run for 2-3 weeks. We collect them out of the tree every night and lock them up. They roost higher every night if I don't find a solution we are going to have to get out the ladder!
They free range in our yard most days and are 14 weeks old. Coop is 3 x 4 x 4 raised about 2' off the ground. We have a 1 1/2" roost inside about 4" above the floor. We will be moving the roost higher and adding another this weekend. Any other ideas are appreciated? See pic of our coop. There is a 2 1/2' wide window in the front (not open in pic) and vents on the other sides and at the top where the roof meets the walls.