With the reduced hours of daylight each day, it's possible that your pullets have delayed the start of laying. Have you checked their vents to determine if they are, in fact, laying now?
If your chickens are laying out in their free-range area somewhere, it is quite possible that predators are getting the eggs, so it's quite likely there won't be an accumulation of eggs somewhere. Laying eggs won't 'make' them go broody. It's a hormone thing that can occur no matter where they lay. It's an instinctive thing to want to lay in a cozy hiding place, so if you want to look around for nesting spots, look in and under things. One member (I think it was here) had a hen laying in a hollow tree. If you can take the time to do so, hang out with the flock and then follow pullets when they strike off on their own away from the flock. She might lead you to her secret spot.
As Gary said, keeping them contained for a week or so will encourage them to use the nest boxes. Scrutinize your nest boxes for suitability. Is it possible the pullets have looked for an alternative place to lay because the nest boxes are not to their liking? (too small? infested? too few? no privacy?)
