Quoted from Nihonmasa:
I will try but the manual specifically says 4 balls, and with only 3 balls the switch won't be down
Was purely mechanical...
The rod in this popper get screwed into the plastic assy.
Mine had became substantially loose, making it sit lower than expected (for the rod part) => coil was not able to pop it up.
Screwed it back all the way up => works like a charm
In case someone has the same issue in the future, to avoid spending hours looking elsewhere