Why does my bobbin wind unevenly?

An unevenly wound bobbin — thread piled at one end, or loose and loopy — causes tension faults that get blamed on the machine.

The usual cause: the thread is not passing through the bobbin winder tension guide. This is a separate guide from the main threading path, usually a disc or a hook near the winder spindle, and it is easy to skip because the bobbin still winds without it. It just winds badly.

Check the thread path for the winder specifically in your manual. It is not the same as the sewing thread path on most machines.

Wind at a steady moderate speed. Winding at full speed piles the thread unevenly and can stretch synthetic thread, which then relaxes in the bobbin and pays out inconsistently.

Start with the tail held and trim it after the first few turns, so it does not get wound into the middle of the bobbin.

Do not wind over an existing partial bobbin. The new thread does not lie evenly over the old, and if the two are different threads the tension changes mid-seam for no visible reason.

And use the right bobbin class. A bobbin that is fractionally wrong for the machine sits at the wrong height and produces looping that no tension adjustment will cure.