Eldritch starting Disco Td5

Hi fellas, can anyone enlight me on this: After a cold night if i wait untill the glow plug indicator estinguishes the engine will turn and turn and turn....and nothing. No matter how cold it is if i just suddenly turn the ignition(like on a petrol one).....it starts without probs(max 6 turns when very cold).....its inexplicable for me.....maybe one of u will help me on this....cheers

aaaaaa.....no fault codes on Hawkeye