They are free ranged,sometimes they get table scraps, but more often they get layer feed and scratch. Could they ALL be in a slow molt? I'm at a loss.
Have they been wormed lately, checked for lice? Do you dust the nest boxes? They won't get in the nest boxes if there is something they don't like about them. It could be bedding, too much light, lice/mites hiding in the boxes, being harrassed when trying to lay, etc. I've seen all kinds of weird behavior when it comes to laying and nest boxes. It's like Goldielocks and The Three Bears.