Have you checked your engine compression? Also do you have a block heater?
The only way I've been able to get this engine to fire without a block heater under 30 degrees is to crank the starter for at least 30 seconds until the engine coughs to life, hence why I leave it plugged in.
With a block heater and the engine warm I let the glow plugs complete a full cycle then, I start another cycle and once the lights goes out I start the car right up, as if it were a summer day.

How is your starter, and what is the battery life?

__________________
Nosce Te Ipsum "Know thyself"