Having had my fair share of problems with my oil burner I can suggest one or more of the following posibilities.
- ran out of oil
- in-line filter blocked
- (may not have one) second filter inside boiler (just before diesel gets to ignition stage) blocked.
- low water pressure
- depending on location of tanks + boiler + route of pipework - the diesel gets too cold and becomes a little jelly like, therefore restricting flow.
- crap in the firing nozzle.
- (unlikely, but possible) soot in boiler / chimney. Keep it clean
Remember a couple of points. A yearly service should keep most of these issues at bay. If you haven't had it serviced in a while, then there's a problem straight away.
Boilers tend to start going on the blink around now, as thru the summer they work very little and then suddenly huge demands are placed on them. And as their use in summer is limited, then winter use, huge, it reacts similar to a car engine. Potter around town for 000s of miles, then blast it out on the M-way for long periods, you're gonna gets problems.
Moral being, that service time is good in autumn, just before long periods of use.
HTH