Firstly I would check the voltage on the glow plug relay and then each of the glow plugs individually to make sure they are getting power. You can take them out to see if they have carbon build up on them and test them (be careful they will get VERY hot). They should be on the front on the engine and will have pipes going to the injector pump.
There are many things it could be, my problem was air getting into the fuel system from the auto choke that diverted the cold diesel to the thermostat housing to warm it up initially this was on a saxo though. It would start after a few goes and it gradually got worse until it wouldn't start this was over the space of a week so check for air/fuel leaks. It could also be that the injector pump timing has skipped a tooth (unlikely), valve clearances, dieing injector pump, dead fuel pump, blocked fuel filter etc
Is the fuel pump priming?
Start cheap and get some new glow plugs, try and get OEM ones as aftermarket ones last no time at all and usually are crap from what I have found. Check the injector pump timing also.
As you probably know diesel combustion is based on compression and not a spark so I would do a compression test on it to see if there are any problems there next.