Small Appliance Repair Questions? Ask a Technician ASAP
There is really nothing complex about winding the bobbin on the 9400.
It passes thru the tensioner as with any other machine, to create the correct degree of 'tight' on the bobbin
Wondering if you have maybe missed something in the setup of the process...
If you are happy that you are doing it correctly, perhaps you could upload a short video so I can see what it all looks like ?It would be almost without precedent to have the winder tension be at fault here, Janome QC is very thorough and issues with new machines are so rare as to be vanishingly unlikely... however, a call to arrange a meet up at the store where it was bought may be necessary to have any warranty issue resolved