2006 f650 what is causing the glow plug light to only come on some of the time.. when it comes on, the truck starts.. when the light doesn't come on, the truck will not start.

SOURCE: No glow plug light, car turns over but won't start, no CEL or codes

I don't have the solution but a possible help in diagnosing.  If you remove the water temperature sensor connector from the sensor, the glow plug light should stay on when you go to start the car.  Try leaving it on for 15 seconds and then start the car.  If it starts right away, it is your sensor.  If it doesn't, the good luck.  
The sensor is located on the right side of the engine near the top, at the back.  It is in the coolant fitting directly behind the hose from the rad, just left of the thermostat.  Be careful to disconnect the wires, and not remove the sensor, or you will loose a lot of coolant.  The sensor is held in by a separate plastic clip, and the harness just has a little built in clip on the side.  Hope you have small hands...

SOURCE: TDI Hard Start

This could be an Injector But, Not likely. Try this First, It is a long shot But, It is Cheaper. Replace your Gas cap. You could be Sucking air into the Tank from a Bad Gasket on the Gas Cap Causing your Hard Start.

SOURCE: 1991 f350 7.3 diesel hard starting in am

you will find that it may be the glow plug timer is faultycausing the glow plugs not getting hot enough

SOURCE: had problems with vw caddy van starting when warm,...

2004 vw 2000sdi caddy van wont start some mornings you need to wind it over before it fires, immediately after it fires you can switch off ignition and it will start first time and continue to start all day. vw dealer says no fault codes present. gloplugs renewed, new battery,fuel problem? Has anyone come accross similar problem I

SOURCE: 2004 landcrusier 100 series. glow

THIS COMES ALL FROM "WAIT TO START" DO YOU WAIT TO START? IF NOT,THEN THE GLOW PLUGS ARE BUR'NT OUT,WHICH YOU WILL NOT NEED UNTIL FALL. OR MAYBE YOU'VE GOT IT HOOKED ON EITHER,STARTING FLUID,IN WHICH CASE,I SUGGEST YOU REPLACE THOSE GLOW PLUGS NOW.OR ELSE YOU'RE MOVING ON TO EVEN WORSE ISSUES.LET ME ASK YOU THIS,WHY ARE'NT THE GLOW PLUGS BEING DENIED POWER? AFTER ALL THEY START THE ENGINE? AFTER YOU INSERT THE KEY INTO THE SWITCH,WAIT TO START,LIGHTS UP.IF YOU'VE NOT WAITED UNTIL THAT WAIT TO START LIGHT GOES OFF,,,YOU'VE BURNT THE GLOW PLUGS UP.THE ONLY THING TO DO IS REPLACE THEM ALL.IF YOU'VE BEEN USING STARTING FLUID OR EITHER TO START YOUR ENGINE FOR A LONG PERIOD OF TIME,FORGET REPLACING THE GLOW PLUGS,BECAUSE YOUR ENGINE IS ADDICTED TO THE BOTTLE(EITHER) YOU MAY BE ONE OF THE LUCKY ONES,REPLACING THE GLOW PLUGS MAY RESOLVE YOUR ISSUES.BUT,WAIT TO START,,AIN'T NO JOKE. BEST OF LUCK TO YOU & YOURS!