Leaky tires happen. Sometimes you can just look at them and find a nail or screw stuck in them. Sometimes it takes spraying soapy water on them to find the puncture or the leaking around the bead seating surface.
For those times that even soapy water fails you, there’s always the dunk tank. Just put the wheel in and fill it with water and look for bubbles leading you to where the air is leaking. I never like having to do this, because it tends to take a long time, and sometimes I spend an hour staring at it without seeing a single bubble.
Mechanical Malarkey stickers available for $5 anywhere in the US!