Glow plugs, Do you have a ohm meter, Check the resistance on each if they are out you should not get a good ohm reading between the lead and the block. Like an open circuit, I have had them burnt out by being held on to long.
Another thing to check is have someone turn it over as you crack the line lose on one of the injectors, does it spray out good. If you do not get a good squirt each time it is supposed to then you have a problem like air in the system, plugged input or something. If getting good flow at the injector connection, and the glow plugs have good resistance, then start looking at something like compression or timing. Had a Mercedes that jumped timing one time. Not fun.