There's not really anything you can do but wait, easier said than done I know.
where are you at? they could be a little late because of the cold.
my experience is that there really isn't a magic number of weeks till they "should" be laying. mine started anywhere from 21 to 27 weeks, and it took a while to work out all the kinks like fart eggs, soft shells, not laying in the boxes, etc.
some things to look for, their combs and wattles turn redder right before they start to lay, some start squatting when approached, and a for-sure sign that you need to check the nesting boxes is when you hear the egg song(bok bok bok bok bok bok bok bok BGAAWK!)
and mine, for some reason started acting frantic around the coop(they're free rangers) like they'd be doing the egg song and try to jump into the coop window(it's plexiglass), jump on top of the coop, it was almost like they couldn't figure out how to get in there, even though they'd been going in and out for months....chickens