Does your truck have a catalytic converter? If so a partial blockage could give you those symptoms.
Next time you try to start (from cold!) have someone put their hand over the exhaust. If there is hardly anything coming out when it fires up your exhaust may be partially blocked.
 
TD5 does need a fully charged battery to start. If its been stood for days it may not be upto the job, easiest thing to try first is charge the battery.
If battery is low it spins over but just has not got enough oomph to kick into life.
 
As above if voltage drops below 10.5(I think) during cranking then the injectors won't fire
 

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