It may be the safety valve in one of the pigtails. Turn off all your tanks, disconnect both the pigtails, then reinstall them and turn on your tanks SLOWLY !!!!! There is a safety valve in the pigtails and a large leak valve in the tanks that will activate if the flow of propane is "too great". The purpose is to prevent a major gas leak in the event of an accident, but if you just hook up a tank and rapidly open the valve, you may activate one of the safety features.
To reset them, you have to "deactivate" them by disconnecting and starting over. It sounds like your first tank was working OK, but when the autochange regulator switched to the full tank, something triggered a safetly valve. It may have been triggered when it was initially turned on "back when"
Hopefully this will get you going again. It's most likely not a problem, but a design safety nuisance