It sounds to me as if your top thread is too tight, therefore, not being pulled to the middle of the thickness you are stitching. If the top tension is set above zero, you might consider giving it just a nudge toward zero. At the same time, tighten the bobbin tension 1/4 turn. Always adjust the top tension with the presser foot down.
If the top tension is 4 or 5 above zero, turn it at least 2 numbers toward zero and try this before doing anything to the bobbin tension.
As you pull both threads from the threaded machine with your hand (presser foot up), they should both pull with the same amount of tension.
Hope this helps.