Restricted propane Flow

I had the same problem with mine Pete, I have two 40 gallon propane tanks, you can switch sides just like yours, my selector switch/regulator is located on the drivers side(Left side) of the 5er, the other tank located on the curb side, (right side) also has a red regulator....When I turned the furnace on, using the left side propane tank, there wasn't enough pressure to keep it lit, though it would have enough to cook with, to run hot water heater and to run the refrigerator. I checked hoses for obstructions, found none..replaced the red regulator on the side that couldn't keep the furnace lit....it didn't help..SOOOO, I replaced the regulator on the other where you select what tank you want to use, and BINGO...problem solved :).....Ever since I replaced the combination selector valve/regulator on the drivers side of the 5er, either tank will now keep the furnace going. That was my experience...