The problem is probably poor compression due to failure to adjust the valves this fall. Valves should be adjusted every fall before the onset of cold weather, or every 12.5k or 15k miles (depending on your car - read the owner's manual), which ever comes first.

The best way to verify proper glow plug operation, if you have the parallel "pencil" style, is to measure the current flow to each plug. One way to do so is to get an automotive ammeter gauge meant for dash mounting, preferably with a 30 amp scale and some markings so you can see rough amps (i.e. mark for 10 and 20 amps, or every five amps), and energize each plug through the meter. Amps should initially spike at about 20 or so, then fall off to about 8 as the plug warms up.

Another way to test is pull each plug, and apply power using a set of jumper cables, be VERY careful as the plug tip will glow white hot at ~1500 deg F.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC