Many of these pressure washers use a cam to run the pistons, the only thing that keeps the pistons against the cam is water pressure, either created by the backpressure of the pumped water or the supply pressure from the garden hose. If the pump is spun without water supply pressure the pistons will hang up in their bores. You need to purge all air from pump through open nozzle, once it quits spitting air then close the gun. Let the trigger slam shut several times , this shock will drive the pistons back to the cam lobes.
Also make sure that any soap injector system is blocked off, they are a source of air draw if not working right.