You would have to think that it was impacted by something, either a foreign object floating about the combustion chamber (shudders in absolute horror!
) or the plug was inadvertently knocked whilst installing (hopefully the case)
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"
'80 Triumph TR7, '73 Land Rover (Ford 351. V8),
'97 Ford Falcon Longreach 'S' ute,
'98 MG-F, '69 Ford F250.