Make sure the nestbox has damp warm 86 degree sand. If conditions are not right, somethimes they just won't lay their eggs.
A radiant heat panel with a good thermostat can be used to insure the right temp of the sand. When used, Cornuta would lay their eggs directly underneath the radiant heat panel 100% of the time. The one exception was the time the panel was accidentally unplugged --the eggs were in a corner instead. The one pictured below layed eggs every other year. During the years she did not lay, she would look semi-gravid in the summer, then shrink back to normal during the fall.
Good luck