The problem is there are a few things that will cause this. Moving the fabric too fast can cause this. But also pulling on the fabric so the stitch isn't formed properly can to. You can change the needle to the proper sized one for free motion and clean all of the lint out of the bobbin case and under the needle plate. If that doesn't work, I found sometimes the bobbin thread is the culpret. Or that you don't have the right bobbin in there. Mine comes with two and it is useful to thread it through the little spring loop. Change bobbins with another thread. Bernina's are famous for not liking some threads like coats and clark.