I've had some breeders who were very stubborn about not laying in the lay box. Sometimes they're laid outside the box because they're not fertile, but other times they just don't get it. As I see it, here are your options:
--check often, catch the eggs soon after they're laid and incubate
--keep putting the gecko in the lay box, several times a day if necessary, and hope she gets it
--put the female in a small enclosure that's completely covered with laying substrate, a sort of huge lay box and let her lay there.