Remember that when the bottom thread is flat, you tighten the top tension OR loosen the bobbin tension. When the top thread is straight, you tighten the bobbin tension OR loosen the top tension. The tension is made adjustable so that you can adjust it for different threads and fabrics. So play with it.
Also, always remember to thread the upper spool with the presser foot up. This opens the tension disks so the thread can get down into them.
Thread quality is also important for good tension. At the shop, we never could get a balanced tension with Coats and Clark or the 10 for a $1 threads, especially in the bobbin. We always used Gutermann or Metler threads to balance the tensions.
Good Luck!