ktbb

Are you by any chance using a plastic bobbin? the problem with any stretchy thread is that when winding the bobbin, it stretches as it's wound. Once the winding is complete - the thread tries to return to normal and in doing this, puts pressure on the core of the bobbin. If you used a plastic bobbin, the pressure of the thread can distort the bobbin center and it won't work properly....this can easily cause tension problems. If you used a plastic bobbin, either try a metal one, or wind another bobbin on a slow speed in order to reduce the amount of stretching.