6C plug is good only at lower temperature and high humidity (let's say humidity higher than 60% and temp lower than 20ºC)..........

if the head clearance is too small it's shortening glowplug life (not to mention the engine as well)........

try adding a 0.10mm shim and check the engine..........you also should try 7 and 8 plugs, specially for high nitro content.........