I should ask too, have you picked them up and felt them to see if they feel like they are laying. You should be able to feel a spread of the pelvic bone opening to about 3 finger widths wide. That could give you some idea about whether they are coming into lay or not and if they do feel wide enough, I would look closer for invaders snatching the eggs. Lock you coop too in case it is a 2-legged swiper.