Diagnosis
This is almost always an airlock in the pump or a blocked inlet. The pump can't prime if air’s trapped or water can't get in.
How to Fix It
Purge the airlock. With the engine OFF and water supply ON, squeeze and hold the spray gun trigger. Pull the recoil starter cord 2-3 times. You’ll see water spurt out. Release the trigger, start the engine normally, and it should work.
If That Didn't Work
Check the water inlet filter. Turn off water, disconnect the inlet hose, and look inside the washer's female connector for a small screen. Clear any debris.
Heads Up
Never run the pump dry for more than a few seconds when starting. The internal seals rely on water for lubrication and cooling.