Are the boxes easily accessible? Have you already placed some "bait" (golf balls, wooden eggs, etc) in the nests?
Laying outside the box is pretty common with a new layer, some start out in the boxes from day 1, others take a little time to get the idea of the whole process. The above things - making the boxes easy to get into/out of and having some bait in the boxes will help -- you want to make the boxes seem like the best place to do with instinct is telling them to do - finding a secluded, dark, safe place to lay an egg - the bait eggs tell them that the nest is an approved/safe place to lay because it looks like another hen has already used that place to lay her eggs too.