Hi,

does it splutter, smoke etc as its trying to start, or just seem like
nothings happening at all until it finally fires normally ?
Is your battery suspect ? does it turn over slowly ?

Your options are:

Fuel supply - eg air leak, faulty stop solenoid etc
Battery - Poor battery giving slow cranking, reduced glow plug current
Glow plug relay - could be high resistance contacts etc.
Engine - poor compression

Depending on how the engine behaves as it tries to start, you can make
a better guess at where to look first. Have you tried anything like ;

Manually priming the fuel system before starting
charging the battery overnight or at least for a few hours before
starting - or getting a jump start
Doing a few glow plug cycles before starting

Cheers
Craig