They both look like pullets to me. Pullets don't start laying at the same time, even if they are the same age and breed and bloodlines. As long as they both are acting, eating, drinking normally, you probably just need to be patient. For instance the ISA Brown commercial birds they expect 2% to be laying at 18 weeks and peak at 96% at 27 weeks, so there is a lot variation even in birds that have been strictly controlled so far as diet and housing etc goes and bred for years for to be uniform.
If you let them free range, or have a place a bird could go into hidden, I'd go on an easter egg hunt...somebody may be laying where you least expect it.
Lady of McCamley