Yes, I would say it's a tension issue. If the problem's on the top, it usually is a bobbin issue. If the problem's on the back it's usually a top thread issue. Are you using the prewound embroidery bobbins? If so, this thread is so thin that you may need to tighten your bobbin tension just a bit. Before you do anything to your bobbin case, make sure you mark the current screw position with a permanent marker. Make TINY adjustments.