Hello, there are multiple options in main jets ( nozzles) if the nozzle is not atomizing properly, the spark ignition will not engage its spray pattern and you will not get ignition. That is just one aspect of the issue.
Another aspect of the problem is oil pressure, that can be set to high or too low for the nozzle selected (nozzle selection depends on the heating unit capacity, shape of the combustion chamber, condition of any deflector plates and clogged vent issues or not.
The other aspect is the CAD cell 'electronic eye' that senses flame, it needs to be working properly along with 4 inch cubed shaped ignition relay that it operates. A dirty CAD cell will also cause this problem.
It sounds like you are getting a thorough service judging by the fact of their changing the nozzle...the diagnosis is beyond what most home owners have the skill to cope with.
I can still help you however, first step is to get a written report from the service company... ask for the following:
1. Smoke test results
2...Flue gas analysis report, percent oxygen, percent Carbon Monoxide.
3. Nozzle brand and ID numbers (so we can determine if it is correct or not).
4...Spark ignition gap.
5. CAD cell light test results.
Tell me what they say to you *verbatim, their response will let me know what class of company we are dealing with and what range of mistakes they made when doing the service.