I have, and have had, several Janome machines. Haven't seen the Harmony, but all my machines prefer Janome bobbins. If you look carefully at the spindle in the middle of the bobbin, Janome bobbins have a "J" engraved on the spindle where the thread winds. While "15" bobbins might look the same, Janome bobbins have some ingredient besides the plastic. Have you tried more than one bobbin? Different threads? Good luck. Ann