I was printing fine lately with every print coming out exactly how I wanted but I've run into a snag again.. half way through the last 2 prints, the extruder stopped extruding. Then when I cancelled the prints and tried a 'load filament', still there was no extrusion. I could push filament through ok but it wouldn't push it through itself. Any ideas of how to rectify this? The motor is turning.. I'm using stock extruders as the recommended spring ones don't appear to be in stock.