Oil burner will not light in the morning, I have to press the reset
button, then it fires right up. It then runs all day without any
problems. The technicians have worked on it three times, and have not
replaced any parts or found any problems!
What are the most likley causes of this type of oil burner problem?

Perhaps, the morning temperatures are very low. This makes oil very
thick and so oil can not reach the burner. Once the burner starts up,
the oil is heated well and especially the temperature is also going up
and so there is no problem. In all such cases, they have a pre-heating
to make oil thin. Try that.