Usually because the water in the boiler is hot enough  normally because hot water is now coming back from the rads, but if the circulation is not occurring, then maybe it gets just a boiler full of water hot and then stops, or perhaps the temperature sensor in the boiler is defective and 'thinks' it is hotter than it really is.

Or some other sensor in the boiler is indicating a fault - often there is a 'flame out' detector that is a photocell and that can be fooled if it gets sooty.


So, is the boiler hot, are the pipes hot?, is the flame nice and bright. 

And do not overlook the obvious -  has the room thermostat or the timer been set to minimum  ?

is the oil present and not wet or bubbly so the flame is unstable.

Is there water in the system, is the pump turning.....

I’ve turned the stat full up so it’s calling g heat. 


pipes are all hot?

 

I assume from you saying the water in boiler is heated so no need it to constantly burn

Is there a water temperature thermostat on the boiler?  If it's set too low, the boiler may have decided that the water's hot enough, so it then turns off until the water has cooled down again.