Oil & Fuel 6x4 Gator Diesel

[KDOregon]

I've got a gator diesel, the glow plug light will not come on (thus it will not start). I've replaced the glow plug relay and the ignition switch, but still no luck. It turns over fine but won't start - midday, when it's warm, the engine will fire briefly but will not continue to run - any ideas?

Thanks
KDoregon