You need to get it scanned for faults.
A faulty glowplug circuit will show up and tell you which one
It may not be glow plug, ic could be head gasket, porous cylinder head...
putting a small amount of water in one of the cylinders, causing rough cold start & idle.
glow plugs don't work above a certain temperature (think about 6 degrees).
When they are needed, if one isn't working and the car starts, it'd be with the rest in less than 30 seconds!