I would verify if the manifold plug is even a viable option as it's possible that plug doesn't go all the way into the manifold. It could also be that something in the manifold is blocking the propane from flowing.
Can you remove your added supply line and briefly turn on the tank and see if Propane flows out of the previously plugged hole? If no propane then see if you can find a way to view what's inside the hole in the manifold. Depending on the location you might use a small mirror and flashlight to see inside. Once you can verify that you have propane flowing out of the manifold hole you can hook up the line and troubleshoot further downstream.