Did your service people check the photo cell to see if it was dirty or not working? This will shut your burner down if not working properly or dirty. It will startup and run for an extended period of time but will shut down if it doesn’t see a flame.
If your burner is running rich it will clog up the photocell eye and shut the burner down. You can tell if your burner is running rich by looking at the nozzle and the cage around the nozzle. If they are black with
soot it could cause a problem with the eye.
As you know the way oil burners work is oil is sprayed through a nozzle at high pressure (at least 100 psig) and is ignited by an electric spark from a high voltage transformer.
Once the oil starts burning there is either a photo cell to detect light from the flame or a thermostat in the stack (chimney) that detects heat from the fire and needs to see it within a certain period of time (usually 45) seconds or it will shut the burner off.
If the flame is not established the circuitry on the control properly called the "Primary" will activate an electric heater on a thermal relay that will pop open after the 45 second time period. After this happens you have to reset the control (after it cools) and you have another 45 seconds.
Newer controls may have an electronic system that gives you like 2 tries to get the oil burner started before it gives up then you must follow a procedure to clear the fault.