Is it nylon monofilament? If so, you do have to make some changes to tension. Was your practice piece not using monofilament? You really need to work out the tension on a test piece using the same thread you want to use on your quilt.
Nylon monofilament thread up above requires a lowered tension setting. (I think this applies also to polyester monofilament.) This is because nylon monofilament thread stretches. You lower the tension so it doesn't stretch so much when it is going through the needle. Stretching causes the thread to break.
Did you bring up the bobbin thread to the top at the beginning of your stitching line? And hold both threads when you started? That prevents a thread nest at the beginning.
Are you using monofilament in the bobbin? At least for nylon monofilament, it is important not to stretch the thread when winding the bobbin. The bobbin should be wound relatively slowly and carefully. Also, it's important not to overfill the bobbin; you want to stop when it is about 3/4ths full. This is because monofilament stretching can cause distortion in the bobbin. Plastic bobbins are more likely to distort than metal bobbins.
What brand is your monofilament thread? Not all monofilament threads are good for quilting.