This is what I was going to say. Getting up is easy, but they need enough space in front of the perch as they fly out and down to make a soft landing. I couldn't remember the rule of thumb, but if
@jthornton is correct and it's 45 degrees then whatever your perch height is, they need the same distance out in front of the perch as a landing zone on the floor.
As others have mentioned, it definitely takes time until they start roosting. Maybe wait a few weeks and see if they get it figured out before making major modifications.
Your nest boxes are designed in a way that the birds can roost on top of them, so as long as the roosts are higher than the floor of the nest boxes, they should choose to sleep there.
Good luck! Good job on your coop build!