You are drawing more gas than can boil off to vapor. Small tanks are very prone to freeze up. Not enough surface area on the liquid to allow boiling of to vapor. You may need to install a magnetic heater to the tank to help with it to vaporize the liquid. Propane boils at -44*, cooler the ambient temperature is, the slower propane will boil off. If the tank is very cold and has condensation or frost on it, chances are it is too cool for the propane to boil off and maintain the pressure for the draw required.