Spray pattern of injectors may be dirty, instead of a fine mist in a V pattern they shoot or dribble droplets or a stream.
Also the delivery system must be tight with no leaks, no air coming into the fuel stream. Verify fuel pressure. Fuel filter must be clean. If it is very cold where you are a 10% gasoline can be added to winterize diesel but no more.
If there are glow plugs verify they all work, even 1 failed glow plug can make starting a diesel tough.
Remember that compression, a very high compression is used to ignite the fuel/air mixture, if compression is failing due to age, use compression becomes too low to squeeze the air/fuel mixture. The injectors atomize the liquid and feed the right proportions of fuel and air, if anything is wrong the mixture is tougher to ignite. Heat of compression starts the banging.