If I remember correctly those are Bosch nozzles. They stick the fuel spray pattern gets bad, gets hard to start blows black smoke. Eventually, it will get harder to start, then begin blowing white smoke. Glow plugs won't help. Those nozzles are famous for it. Remove them and have injection shop test them. Not too hard to remove I made a deep well socket just for that nozzle, they are real common nozzle. Hardest part is breaking the injection line loose use a good wrench. last thing you want to do is round that nut over. You may also have to hold the nozzle body from turning. When you remove them there is a brass washer, it may or may not come out with the nozzle. So, you might have to use a o ring pick to get the brass washer out. Those have to be replaced. The injection shop will have those and the rubber/cloth fuel return line. Chances are you will have to cut that. I would not count on injector cleaner working and i don't believe in the stuff. It is a pintle style nozzle, and real susceptible to heat, over time the start to stick. Why they all start doing it at once never been able to figure out for sure. I have seen it where if a machine gets real hot to the point of almost over heating, then they shut machine down real fast. had them do it. You replace the customer's water pump he starts going nuts because his water pump failed got hot and now that that is fixed it is blowing all kinds of black smoke and hard to start. then you tell him his nozzles need to be rebuilt, sometimes its a tuff sell.
