If the other plugs were 'wet'/used looking from firing, and that plug was dry, there can't be any fuel getting to that cylinder, hence the inlet valve can't be opening. That, or something's been dropped through the carb and is diverting/blocking the fuel away from the manifold header relevant to that cylinder.
That would have to be some cam/follower wear to not allow the valve to open, but anything's possible with these engines!
Easiest way to check is to simply remove the rocker cover, and start the engine. Keep an eye on the inlet valve of the suspect cylinder to see if it moves/opens as much as the other inlet valves.