I answered this same question yesterday to someone else. These exchange places don't purge their tanks properly when they refurbish them to bring the tank to code. There is moisture in the tank that has frozen when the exchange place first filled the tank. It looks like snow floating on water. When you create a vacume in the tank by cooking, it draws that ice into the regulator and packs the inlet so no gas can pass. The only cure is to add methanol to the tank but because you'll only get that at a local propane co., your best bet is to buy a new tank, have it purged properly before filling it. You must introduce propane vapor(not liquid) into the tank and bleed it out to remove any air. Propane is heavier than air and will take the place of air in the cylinder. You must do this 5 times before filling it with liquid. Sparky at your local agway is clueless about this even though the propane supplier has trained the owner and a few employees on proper filling.