Do you have access to dirt in your back yard that does not have a lot of fertilizer? I will dig a small amount up where I know it does not have pesticide or fertilizer and use that along with play sand. Another thing that has helped my female lay in her bin was using damp sphagnum moss ontop of the soil/sand mix. I put the moss in a plastic bag, ran hot water from the tap into the bag, drained the excess water, and placed the moss loosely on top of the soil in the bin. She seemed to like it because she dug a hole and laid her eggs the next morning.