Are you keeping you top and bobbin threads close in color and value. I will not quilt for someone if they insist on something like white on top with red bobbin. Increasing your top tension slightly may solve it. Change in Very small increments and write down every change so you have a record. Remember changes in thread & batting will change tension requirements as well. I put a dot on my tension knob and keep a notebook listing how many turns less or more I need for each type of thread. I rarely change bobbin tension. Superior threads sells a gauge that tells you your bobbin tension. I use 3 different threads for bobbins. I have a separate bobbin case for each type. I used the bobbin tester to change them all to the same setting. Now when I change thread I only have to adjust top thread.