The holy grail of bees.
Consider that 70% of bees live underground.
Well.... where are they??
Ground-nesting bees can be pretty hard to find. Most are dispersed in solitary holes that are well-hidden... unless you see a bee coming or going, the holes are invisible.
Unless of course, you are down on the ground like a bee nerd, looking for them.
To encourage ground-nesting bees in your yard, keep some patches of well-drained soil with sun exposure BARE and UNDISTURBED - no tilling and no mulch.
And most importantly... DO NOT USE PESTICIDES IN YOUR YARD. Consider alternatives.