Instead of replacing all of the glow plugs to try to correct your problem I suggest that you do some simple trouble shooting first.

Note : The glow plugs are connected in parallel within the glow plug control box.

This is for a 82 300SD, it should be similar for other years.

On the drivers side wheel well, disconnect the connector which connects the control module to the glow plugs. Do a resistance test between each of the 5 contacts of the connector which go to the glow plugs and the block of the engine. The resistance of new glow plugs is 0.4 and 0.5 ohms. If any are greater than 1 ohm or so I replace them.
At this point, you will know if you have any bad plugs and how many. Do NOT reconnect the glow plug connector. If you have any bad ones, do a resistance measurement from the power connector of each glow plug to the engine block to determine which plugs are bad.
Reconnect the glow plug connector.