could be air leaking in through the balance pipes of the injectors try parking it on a hill with the front facing down the hill with at least a 1/4 of a tank of fuel.

If one of the balance pipes has a small hole in it over night as the diesel returns to the tank and the diesel comes out of the balance pipes it drags in air and can be right bitch to start in the morning.(it happened to mine)

you can hear if the glow plug relay is working if you turn the ignition on(but don't start it) wait for the light to go out and listen real carefully you should be able to hear the relay click off after about 10-15 seconds after the glowplug ligt has gone out.
If you hear it there still may be a problem between the relay and the plugs tho but at least you'll no the relay is switching.