1984 Nissan Patrol problems starting the engine

1984 Nissan Patrol 3.3 Diesel mileage: 125,000. I'm having problems starting the engine in the mornings, especially when it is cold. The battery is relatively new and is charging up ok. The engine turns over and will sometimes start after a few minutes and sometimes not at all. Once running, the engine runs smoothly and will restart with no trouble. I don't know a much about diesel engines and would be very grateful for any advice you may be able to give.