Are your nestboxes higher than your roosts? Chickens instinctively want to go as high as possible to roost. If your nestboxes are higher, that's where they'll end up.
Another possibility - have you checked for mites? Many mites live in the crevices that joins the roosts and come out at night to feed. If they are being tortured by mites when they are on the roosts at night, they'll look elsewhere for a place to sleep.
Another possibility that just occured to me - what are your roosts made of? Chickens prefer a flat surface to roost on. They don't want to grasp with their feet all night like a caged bird would. They use their feet to get steady and then release their grasp and rest on their keel bone. I use 2 x 4s with the 4 side up for roosts.