I have a Weil Mclain oil burner hydronic heating system with Aquastat L8124 and R8184 primary. The system will run fine for several "call for heat" cycles but then fail to fire upon a call for heat. When this happens, the red reset buttons on the primary and on the motor DO NOT popup. Upon simply turning the service switch off then on, the burner fires. What could be causing this?
Also note that I initially had an old Honeywell "dial type", two-wire thermostat and this condition was occurring. I changed to a new digital thermostat and the condition still occurs.

Many of these boilers have a stage delay, or a boiler delay. This delay prevents short cycling when different zones call for heat. Review your manual and check your set points and maybe get a tech to tweak those settings. Are you getting any discharge at the relief valve? Monitor it for a few days. The next time your burners don't fire TIME it instead of resetting it. A delay of 3-10 minutes is not at all unusual after several heating cycles, longer may indicate adjustments.

Another thing that comes to mind is your service maintenance. Just from the point of efficiency how often are your burners cleaned? When was the last time? Every fire up and flame out builds up soot residue in oil fired burners. Talk to your tech or service guy about this before you go about any tweaking yourself is my recommendation.

I highly suspect YOU are the cause of this condition. Monitoring your system and learn the nuances of electronic heating controls is indicated. Let me know what you find.