Breed of birds, time or year (they take longer to start laying eggs if they mature during the winter months when days are short), geographical location and hours of daylight all tie into that. In general, birds maturing in the spring/summer will come into lay sooner than birds maturing in the winter. Sex links and leghorns will usually mature faster than heavier breeds like Light Sussex or Brahmas etc. Some people believe that switching to a layer feed before they start laying will slow them down.... grower usually has a higher protein content than layer feed. Many manufacturers recommend changing to layer at 16 weeks, but that might mean they are on layer for another 12-20 weeks before they actually start laying and the lower protein can slow their development.
Another possibility is that they are laying somewhere other than the nest boxes and they have a stash of eggs somewhere. They can be quite creative about where they lay their eggs. Have their combs turned bright red yet? Are they scratching the nesting material out of the nest boxes? Have you heard them singing the egg song?Do any of them squat when you approach them? These are things to tell you that eggs will be produced soon.