When upper and lower thread tensions are balanced, knot between top and bottom threads is hidden between fabric layers When lower tension is too loose (or upper tension is too tight), knot is visible on right side of the fabric When upper tension is too loose (or lower tension is too tight), knot is visible on wrong side of the fabric. Perhaps you are now using two different color threads and it is more noticeable. Try using the same color thread and see if you stil notice the thread showing on the top. Yes, and clean out your bobbin area it sure could be that dust bunny in there.