Dale W;
Have valve adjustments been checked, is the pump's number one been spill tested for injection timing, what diesel engine are you dealing with? A Nissan engine is always preheated before it's to be turned over. What is the cranking speed of the engine and do we have a load on it while it's cranking (HYD). A previous post asked about the exhaust smoke's color. Has the engine's head been worked on and what other repairs were dome on the power unit just before this issue started? I've seen a few strange reasons for a diesel not starting, and the one of the strangest one was a gas crank used in place of a diesel crank. The first step to take now is to determine what the cylinder combustion pressure readings are vs what they should be.

What colour is the exhaust smoke, is it black or blue/white would you say the running is erratic,

Some possible areas to check defective stop unit,
Restriction on induction system
Restriction in fuel pipe is it a banjo fitting onto the fuel filter feed pipe dirt can collect at this point.
Sticking valves , broken springs incorrect valve tip clearance
Defective injectors incorrect pump timing
Poor compression.
Hope there's some ideas for you to check out

Regards Titus

did you bleed out the injection pump after reinstalling it?
then bleed the injectors?

any air in the system will make the engine run badly and smoke

it was just kinda hard to start before the fuel system was gone through but now it barely runs

How about a bad piston/rings ?