. re-thread your machine with a rainbow of colors. (important that you remove the existing threads if you are not sure it is threaded properly.) make sure you are lifting the presser foot while rethreading.
Sew a good 6 inches.
Look to see which threads are the offenders and tighten the co-responding tension knobs a little.
Sew another 6 inches. Examine the result. Adjust tension again if needed.
Work this way until you have the stitch balanced.
Finally when your stitches are perfect. clip the threads at the spools in back. And tie on the coordinating colored thread for your fabric.
Get used to this procedure since it is common to have to adjust tensions when you change fabric. Good luck!
Answers & Comments
You may have it threaded wrong! Normally tightening the top tension solves bottom looping!