The Sears blaster's gun is a piece of junk. I bet if you take the nozzle apart you will see the sand has worn through it. Same thing happened to me. I am surprised you were able to use it for 20 pounds of sand. Get a better gun, but then that means you paid about $50 for a plastic container. Also, if you are going to do major amounts of blasting, I suggest getting carbide nozzle for the gun. Also expensive.