Have you tried swapping the tanks, left for right? The left tank valve may be defective.
Do they both feel about the same weight
? The inoperable tank might not have any gas. It's possible the second tank didn't actually get filled, or they grabbed an empty tank from the rack if you exchanged tanks.
Did you use soap on the rubber hoses? Soap the whole system that is accessible, the entire regulator, valves, etc. Don't forget to test with the valves open so the system is pressurized. For example, I once found a leak in the middle of a brand new, 10' piece of gas pipe. That was after replacing all 12 fittings four times, pressurizing the system then watching the gauge slowly drop to zero each time. This was to be enclosed behind drywall, so the leak had
to be found!
to rinse well because the soap solution is corrosive over a period of time, rust will form and copper or brass turns green. A spray bottle filled with water helps to rinse under and back-sides of things.
Eventually you will
figure it out! In the meantime, I empathize with your frustration.
& Ann K.