What if my chickens don't start using the nesting boxes when they start laying?

To each their own.  During these critical weeks of just beginning to lay, I don't let the pullets wander to places where they can lay their eggs.  Don't know your setup, but they'll get the hang of it all, as times goes along.

I "train" the pullets by doing some things early on.  I'd rather have this be a bit harsh or mechanical, but then?  Done with it and they don't ever seem to lose their training.

I don't let point of lay pullets wander.  Coop or run, that's it, and no place to hide eggs.  

I place nest boxes on the floor, as chickens are ground layers, after-all.

I don't let them out of the coop until mid afternoon.  90% of the birds will lay before 3 pm.

I do put golf balls in the nests as this does seem to work, to some degree.

I do like it when there are older hens mixed in, (although this isn't always the flock make up) as they show the younger ones the ropes, as it were.

Once I am confident they've mastered all this egg laying and really "own" the nest as their place, I gradually grant them more freedom.

I put some wooden eggs in the boxes and 9 times out of 10 they will lay in one of those boxes.  Sometimes they change boxes and start laying in a different one, but it's usually one of the ones with the wooden egg in it.  Today one of mind layed an egg right in the middle of the coop on the floor.  Good thing it didn't break!  A few weeks ago one layed an egg from the roost, so it broke since it had to drop a foot or more.  I had never seen one do that before.  When I let them out it's usually around 2 or 3:00, but sometimes one will lay after that and they have a spot in the yard under the woodbox they will sometimes go.