Differences between plugs
A 'hot' plug looks like a 'cold' one; the extra length on the nose is on the inside.
However, some plugs have a sealing washer at the top of the thread, while others have a tapered 'seat'.
They fit into different types of holes, and must be tightened differently.
Some plugs have a fixed terminal on top and some have terminals that can be unscrewed.
Make sure that a terminal of this type is on tight - use pliers if necessary.