Could be several things.
First, install a brand new needle, perhaps a top-stitch needle will work better because it has a groove in the shaft that protects the thread. Make sure the needle is the appropriate size for the thread you are using--a needle eye that is too small will cause the thread to hang or shred. When the needle penetrates the sandwich, the top thread forms a loop. If the eye is too small, the thread will not come back up through the sandwich, leaving a loop under the fabric.
Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Set the top tension to the midway point and retest your machine on a quilt sandwich made of the same fabric layers as your project.. Tension problems are more evident when sewing multiple layers of fabric, so tweak the tension until the top and bobbin threads meet in the middle of the sandwich.
I don't believe you will have to adjust the bobbin tension, but in case you do, be very careful. Mark the tension screw setting FIRST so you can reverse any changes you make. Bobbin adjustments are made in very SMALL increments. Because I tend to use a lighter weight thread for my quilting, I have a specific bobbin case that is adjusted for my quilt thread and I keep my other bobbin for regular sewing. Then I don't have to keep adjusting the bobbin tension when going from sewing a quilt to something else.
Some machines do better with certain brands of threads than others.
Adjusting Sewing Machine Tension to Prevent Problems
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