Boiler pump working, boiler not igniting-thermocouple or low water pressure?

Seven days ago, I discovered the furnace in my duplex rental property had been turned off at the thermostat (it was "off", should have been in "heat" position). I have hot water baseboard heat, the boiler is about 30-35 years old. It was 38 degrees inside and there had been no heat for 6 days in the building. Five days ago, a plumber was able to work on the furnace, identified 2 places where it was leaking and repaired the line. The overflow tank on the boiler also needed to be replaced. There were a few leaky valves for the water service that he also replaced.

Heat has been on and working for the last 4 days - thermostat set at 67 degrees. First floor radiators work great, four radiators on 2nd floor work fine. Three radiators on 2nd floor and 3rd floor radiator not working. We suspect the pipes are still partially frozen. Getting good pressure (plumber told me it was 11 PSI cold).

Three days ago, my plumber and I met the insurance adjuster at property and the heat didn't kick on and it was 53 inside. My plumber switched thermostat from heat to off, waited a minute, then turned it back to "heat". Boiler and pump both came on.

I've been at rental last two days. When thermostat clicks on, pump goes, but boiler is not firing. Am doing my plumber's trick, turning off heat @ thermostat, then turning it back to heat mode.

Not sure if this is a thermocouple problem or water level problem - either in lines, or in the boiler itself. Seems to me to be an electrical/sensing problem if boiler is not firing correctly, but wanted to check with the 'pros'.

Resetting the thermostat once or twice a day isn't a big deal, but I do need to get this problem fixed. Finally the non-working radiators are still not working; one is showing signs of life (vibration on the line). Been directing 2 space heaters at area where the pipes run up wall (part of wall is exposed, as it's a dropped ceiling. Wondering if having 4 non-working radiators are causing the boiler to misfire/not fire.

Thanks for any help you can provide.