maybe the line to the reservoir is clogged? I haven’t opened one in a while but I know there is a tank where it keeps water cold and adds C02 to it. If you run from the house directly to the machine without an inline filter it’s possible there is a clog in the line somewhere. You can try replacing the tubing from the water input area to the tank.
Just a rough idea though, That’s where I would investigate. A possible reason it could of been intermittent is becasue if the line is partially clogged it would be easier for it to freeze over if the diameter is smaller. You might also be able to just turn it off for a few days to get it cycling again then run vinegar or something through the machine which might erode whatever is clogging the line.
I’m not an expert though, hopefully Matt might pop in here and some point and give better insight.