I wonder .... ... if they free range 8 hours a day, are they really getting all the nutrients they need from their layer feed and the available oyster shell. Maybe they are filling up, not on treats, but on the outdoor buffet? Here's a suggestion you might try for a week or so to see if it helps: keep the chickens confined till about 3 in the afternoon, then let them free range the rest of the day. Also, put golf balls or fake eggs in the nests where you want them to lay, they will lay there when they see those there. Good luck!