Check both to see if they are within the guidelines of your machine. Do not adjust the bobbin thread more than a quarter turn of the screw at one time. After adjusting, check the tension again by sewing out the three Is. Thread type can also affect your tension. If you are using rayon thread, your tensions will have to be a little looser because the tensile strength of rayon is not the same as that of polyester.
Polyester can stand a tighter tension and needs a tighter tension to avoid looping.
Your needle may also affect your tension. If you have a dull needle it may not be picking up the bobbin thread exactly the way it should. Speed can also affect tension. If your tension seems a little too tight, slow the machine down a little and see if this changes it. Also check your needle depth as well as timing of your machine if none of the above adjustments work.