Maybe it has something to do with the shoddy tube clamps on the back of my machine (pictured below). The day our Makerbot arrived, the clamps on the lefthand side (in pic) were snapped. The kids claim it "came that way." I am not sure the kids didn't make it that way in their enthusiastic unpacking. In any case, it has been an adventure, as the left pictured tube (for right extruder) often slips and the filament gets pinched. To date, our best solution has been this rubber band set-up. However, it is clear that it is not suitable for a 13 hour print job, and needs more frequent monitoring.
I removed the filament, reloaded the spindle and was set to try again. All 13 hours of it. But the filament wouldn't extrude. Additional investigation made it clear there was likely a bit of broken filament jamming the opening to the nozzle.