I had the same curious problem with my tanks. The issue seems that if the tanks are overfilled just a little and then you open the valve too quickly the tank safety actives and you get no propane. The safety is used in case you get a blow out in the line so all the propane does not come out in an uncontrolled manner. Open the valve just a little bit to equalize the pressure for a minute then continue to open the valve the rest of the the way or open the valve SLOWLY.
2004 Ford F550 6l Lariat SuperCrew, Fontaine Bed,
Link Systems Air Ride
2011 Sterling 32RL with Michelin XPS tires