DIAGNOSIS This is a failing or failed regulator. The safety device on the propane hose is tripping and restricting gas flow. It's a common issue after a quick startup or if a valve was opened before the tank.
FIX Perform a regulator reset. Turn all control knobs to OFF. Turn the propane tank valve OFF. Disconnect the hose from the tank. Wait 60 seconds. Reconnect the hose to the tank. Now, open the tank valve SLOWLY—take a full 3-4 seconds to open it fully. Then light the grill, starting with the middle burner first.
IF THAT DIDN'T WORK A partially obstructed orifice or burner tube on that specific side could also cause a weak flame. Inspect and clean that burner's gas path.
HEADS UP Always open the tank valve slowly after reconnection. A fast surge of gas is what triggers the regulator's safety lockout.