You have to adjust the bobbin tension when switching to a different weight thread. Did you do the drop test with the new thread in the bobbin case? An andvantage with 15s is you can easily keep an extra bobbin case set for a particular thread you use often. It's often easier to switch bobbin case than fine tuning bobbin tension, and it saves the wear on the adjustment screw. Another option is to use a gauge to measure the bobbin tension on the thread, but in general, with a bit of practice you develop a feel for the tension setting.