You need to first ensure that the glow plug system is working properly. Use a voltmeter to check voltage at every plug. The 12 volts or so will be divided at each plug since this is a series system.
Then, check the compression, you should have about 300PSI at all cylinders.
I am going through all this right now with my daughters car and it points toward a bad tank of fuel. Ensure that your fuel filters are clear.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in