A couple of things come to mind.
1st) On the 2way regulator, there should be an indicator, is the indicator showing red (if both tanks are indeed full, it should not be red)? If it is red try flipping it to the other tank. Does it still show red? Check the hose connection from the tank to the regulator to make sure it is tight and not cross threaded.
2nd) After you check the connection to the tanks, make make sure both valves are turned all the way to the open position on your propane tanks. If not, open all the way.
3rd) purging the line by relighting the device furthest on the line from the regulator, e.g. water heater, furnace, etc, over and over hopefully until it stays lit.