After I got my dual 3D printer head, I was following the instruction manual and on the last page it suggest that “Power off the machine, open the front cover of the module, and press the extruder buckle downwards to expand the dual-gear extruder;”
While opening the cover that the little black gear part separated from the grey front part and exposed a little spring, that fell away.
After spending some time to find it and replace it, I realised that it is easy to loose this little spring as it is not locked between the back of the cover and the black part with the gears.
If it gets lost, there will be no pressure on the gears to trap the filament and moving it on the the hot end.
Hopefully the SM designers will take note and find a way to secure this little spring