Because they aren't on a digitized schedule. There are a lot of factors that can affect whether or not hens lay. Nutrition, how much light they get, whether or not they are molting, stress, disease, or parasites can all affect whether or not a hen will lay. Most likely though your birds are molting again. Are you seeing a lot of feathers around the cage?My 23 coturnix are 8 weeks old and none have laid an egg yet. How come, all males? Their parents were laying at 42 day and on.