What can you do?   Buy the largest tank you can lug around.  As the internal surface area increases the heat sink increases because the metal cylinder has a larger heat reserve available. You could also complain to your retail gas supplier.  Ask them to purchase the gas from a gas wholesaler that adds some methyl alcohol into the propane. Methyl alcohol acts as antifreeze. Wholesale propane gas suppliers in colder climates often do this in the winter months. If that is not feasible then occasionally pour some hot water over the tank valve and propane regulator.  You can try to keep the propane tank and gas regulator in a warm place. The colder the propane tank and gas regulator gets, the more prone the tank and gas regulator will be to experience a freeze up during high propane demands. Lastly, keep a spare propane tank and be prepared to make a switch whenever you sense a decrease in the flow of the propane. Caterers often like to use a manual tank changeover valve to quickly change to a warm, full tank by the simple turn of a valve. After you switchover to the new tank the other frozen tank can have time to warm back up or you can changeout the problem or empty tank.  A freeze up will be more pronounced with a less than half-full or a near-empty propane tank, and occurs more frequently in a smaller 5 gallon propane tanks than a larger propane tank. Experienced caterers commonly use as large of a propane tank as they can and keep a spare tank nearby. The use of a twin stage regulator will certainly help reduce these freeze-ups because the gas expansion process is drawing heat from two separate regulators rather than just one.  A twin stage regulator also delivers a more steady and even supply of gas.

Often the source of the problem is water within the small, portable propane tank.  Water is not supposed to be in the tank, but usually is. The propane inside the tank is in a liquid form.  Propane leaves the tank in a vaporized state. That change of state from liquid to gas requires a considerable amount of heat.The heat to vaporize can only come from the metal tank that contains the gas. If the demand for the propane is very high, the tank and regulator get noticeably colder and colder.  If the demand for gas is very high the vaporized propane delivered will decrease. If there happens to water along with the propane stored in the tank, the water vapor will freeze internally at the tank valve and can even freeze within the regulator and will further reduce or block the flow of propane to the supply hose.  This is noticed more profoundly when the demand for the propane is at a high level and/or the amount of liquid propane left in the tank is at a lower level.  Water does not mix with propane but both propane and water have vapor pressures within the tank.  Moving or shaking the tank is not helpful.

According to the Coleman company there are at least three possible reasons: First is that the propane tank might have been improperly filled.  All tanks must be purged of air before being filled with gas. This purging requires filling with a small amount of gas and then emptying. Propane is heavier than air and will force the air out of the tank during the emptying. Filling with gas then can proceed.  If the tank is not purged then the air is the first gas to exit the tank and the grill will either have no flame or a very low flame for possibly over an hour until the air does fully exit the tank.   A second cause could be automatic activation of a surge protection device within the propane regulator. If you turn on the tank valve before you fully turn off each of the burner knobs on the grill, the surge protector could sense a leak and activate. The fuel flow will be reduced.  The remedy is to turn everything off, disconnect the tank, and reconnect everything before starting over.   A third possible cause might be that the tank was overfilled.  All propane tanks are now fitted with Overfill Protection Devices (OPD) which is designed to be activated by a float valve.  The OPD feature prevents overfilling of the tank by shutting off the valve.  A 20% empty space is necessary to prevent the tank from venting large amounts of propane when the ambient temperature rises.  This OPD can also be inadvertently activated by tilting a very full tank during moving.