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Air leak.

This can be a porous fuel hose, but often a filter / bleed nipple is no longer properly sealed. In the event of an air leak, you can pump up fuel with the handle on the feed pump, if this results in faster starting then you have an air leak.

Air filter clogged

Clean if it is a mesh filter and replace with paper.

Compression too low.

Valve clearance is not good. Worn piston rings / cylinder wall. Burned-in valves, a cause of valve clearance that is not set correctly. Leaking head gasket.

Cold start installation not OK.

1 or more glow plugs are broken and / or preheated too short.

Injector malfunction.

With older engines, the spray pattern may no longer be good, so that the fuel is not atomized correctly and does not ignite completely. The injector must then be overhauled.

Broken fuel pump.

Not much can break on a fuel pump itself, it is a robust part. What is sensitive is the delivery valve on top of the pump that is attached to the injector pipe. There is a valve and a spring in the delivery valve housing and this spring can rust through if there is water in the fuel. This also applies to the tailgate seats which can oxidize. The pressure line to the injector will therefore no longer remain under pressure and the engine will run poorly or not at all, depending on the number of cylinders the engine has.