Let's see.......... Do you have the nests where they are easy to get to? Do you have them layered in hay or shavings so they feel good? Do you have them in a place where they can have some quiet time alone? Do have new birds just introduced to the flock? Have you changed or rearranged the coop recently? There are several factors that cause them to not lay where they are supposed to. I recently added a few new chicks to the flock and my hens are having a hard time remembering to lay in the nest boxes instead of in the corner. Sometimes they do, sometimes not. Rotten old birds!
Can you provide pics of your setup? We can probably give you better feedback if we can actually see their situation. Nest boxes should also be in a dark area, and quiet if possible. You can put golf balls or fake eggs in the nest boxes to encourage them to lay there. The golf balls worked best for me!