Others here have mentioned that the roosts need to be higher than the nesting boxes for the chickens to use them.
I had a couple of pullets that started using my nesting boxes at night. I think they were at the bottom of the pecking order and found them a safe place to nap. I did nothing but clean the mess each morning. By the time they started laying they stopped sleeping in the boxes.
What kind of roosts? What kind of chickens? I have dual-purpose - large birds - so I took down my 2x2's and now have 2x4 roosts so they can rest on the wider side. Come to think of it, my first roost was lower than the boxes. But that was before I had the nest boxes installed. Later, I added a higher roost because it seemed they were too crowded after the nests were in.
Love, Linn B (aka Smart Red) * * * Nesting with 5 Australorp and 5 Lt. Brahma hens plus 'The Count of Monte Cristo' - or Monte, for short - one beautiful, well-behaved, hard-working, Australorp rooster, in south-est, central-est Wisconsin.