Mason bees do a lot better when clay is easily available. Mason bees need clay for nesting. They need clay for plugging up the nest and between compartments. If there is no clay available, bees go elsewhere to nest. If your soil is sandy or very organic, you can make clay available for your bees.
Our present clay is wet so there is no need to add a lot of water to get it to the right consistency. slowly add about 2 table spoons of water to make it into the consistency of ‘playdough’. Then, within about 10′ of your mason bee home, dig a hole the width of a shovel and about a foot deep. Place wet clay into the hole and onto the base of the north facing wall. The soil in the hole will keep clay damp. If the clay looks like it is getting dry, wet the base of the hole and the water will wick into the clay.
In a lot of soils the first layer is organic, and perhaps a foot or so down is the mineral layer, mostly of clay and sand. If your garden has this type of soil, you do not need additional clay.