Ever want to bang your head on the wall. Try finding an MC-4 cable connector on that feels like it is plugged in but is about 1/8th of an inch too short.
This happened to me when setting up an RV solar panel installation. I could never get the charge controller to read above the amperage for a single panel. It was like the second panel was not there at all. Hours and hours testing panels and resetting connectors.
Every time I missed this small piece of plastic, caught on the edge. Just enough to keep the conenction for working.
Lesson learned, hope this RV Solar Tip helps you.