If they're a year old, it could be from molting, then. Very common this time of year. My trick for finding hidden nests is to keep them locked up until later in the afternoon, let them out and watch the most eager ones to get out of the pen. (They're the ones that have been waiting to get out to go to their laying-spot. I swear, some of them cross their legs until they're let out!
) Mine have always been kind of shy, though, and not willing to just flat-out show me where they're laying. I hang back and watch from a distance, just keeping them in sight to see where they go. If hidden nests are the case, you may want to keep them in the coop/run for a few days to recalibrate them to laying in the nests again.