They lay at different times.Sometimes the hatch date can have something to do with it.I have pullets that are 8-10 mos that haven't laid yet,some that have laid a single egg and no more,and some that are laying now and are only a few months old.A lot of times I've seen the males have a lot to do with it. If a hen isn't 'willing' or submissive they seem to take longer. I have 2 Silver laced Old English pullets that are 9 mos old.They were wild as hell.Always had to use a net to catch them(even in cage).A rooster try to breed and they went crazy.Never got an egg from them.Finally I put them in with a rooster I knew would dominate and wouldn't take no for an answer.I wanted to try a blue laced project bird and it came out perfectly that I didn't have a silver laced rooster and no blue hens,so they went in with big daddy who is my older blue .Well I have just started picking eggs from this cage and I now have one pullet who is 'broke in' and will jump on my arm,while the other is still fairly wild. I figure once the nice weather really kicks in and Blue gets going the other will kick in too.BTW.... I have 2 or 3 "Blue Laced" project eggs in the bator and they are forming well!! So hopefully I'll get something out of em!!!
On a side note,I have a couple of seramas that haven't laid eggs and are well over a year.One thing to watch for though is an egg eater!! If someone in there is an eater,you may not even know when they lay,unless you happen to see it quickly.