you may also want to remove the banjo bolts on the fuel lines at the injection pump, they have screens in them that can get clogged over time and can cause this problem too.

We had a similar problem. found the fuel primer above the Fuel filter losing pressure.
Check fuel shut off power supply at bottom of injector pump
Check Manual fuel primer pump
Also rare but we have had contamination( Dirt, Plastic, sludge) in bottom of fuel tank to block off suction line

good luck