There could be a bunch of reasons. Did you get a load of oil recently, did you run the tank empty? Is it igniting at all? Does it ignite and run normally and then time out and shut down? What does the flame look like? What does it sound like? When was the last time it was cleaned? Did you replace the cad cell/photoeye? Is light getting into the area under the ignitor? Ohms reading on the yellow wires to the cad cell/photoeye when it's burning should be around 300 and 10K or at least very high when it's not burning.
Is your ignitor sparking and does it produce a good inch of spark. Look online for that test. The eye can be tested while burning, to eliminate that cause, but you'll have to look online for that test. Have you bled the oil line? Is the oil pump producing enough pressure? Is the nozzle new? Is the fan turning? Is the motor coupler good?
You see...... it's easy when you know where to look.