did you find air in the lines?
be careful though those primers can be funny
ive had a few su ck ( i have to edit this or else the site blocks the word lol) air in because the valve got stuck open

do you have an inline injection pump or a rotary pump?

you might even want to run clear fuel line to the lift pump to make sure youre not suc king air out of the tank

make sure there isnt a blocked return
take the return off the tank and make sure fuel comes out

make sure the shut off solenoid is working and the connections and wiring is all good

im gonna tell you 2 diesel stories now that are important to check yet easily overlooked

1- vented fuel cap
i had a guy call me in for his offroad Liftking (a candian product lol)
he tells me it will shut off and he wont be able to start it for a few days
at this point some other shop had replaced the injectors, lines, injection pump,and lift pump to the tune of about 2000 US$ with no success
it was dead when i got to see it so that helped
after crankin it a few times and it had a solid head of fuel
i open the gas cap and i hear a huge rush of air
im say to the guy "holy sht i found the problem"
i tell the cutomer to start it now
after 2 cranks it starts right up
turns out he had lost the original gas cap and replaced it with a non-vented cap
as the truck draws fuel air has to replace the volume or the vacuum in the tank will build to the point that it will over come the lift pumps ability to draw fuel
20$ cap and 1 hour service call - fixed never had another issue

2 - fuel line blockage
i had a linde diesel that was hard to start and had little or no power in manhattan
luckily that has clear low pressure lines oem
so i noticed bubbles in the line from the tank
after checking and tightening the hose connections and still had the air i just took a walk around the truck
i noticed there was no fuel cap
so i ask the operator what happened to it
he said after they refueled it they ran it over by accident (lol)
so i ask "how do you keep the rain out"?
he says that "we put a plastic bag over it but it must have fallen off"
i say "i dont think it fell off i think its in the tank"
i take the whole tank apart and get the pickup assy out
and sure as sht i can see it has a plastic bag stuck about a foot up an 8mm ID metal tube
3 hours and 1 sacrificed metal coat hanger later that truck ran great
and still does he just traded it in for a new linde diesel

check those things out
try to get there when its acting up
and ask him to notice if he gets a noticable rush of air when he refuels the machine or if it has an after market cap on it

the tank cap part# should be 9136410600 but have that double checked by serial number
sometimes the vents go bad or get clogged so clean or replace it
its pretty cheap (25$ list) and you can UPS it to the customer lol

if all else fails you can pull the pump and have it bench tested at a diesel fuel shop and have the injectors pop tested

but make sure you line up all those timing marks b4 you pull it

also you may want to take samples of his fuel and send them out