Only my option. I think it is your machine!
The stitches on the back should be the same on both sides on the back side, yours are not.
If the bobbin thread is to loose, your top thread will put the bobbin thread up to the top, evenly on both sides of the zigzag.
If your top thread is to loose, the bobbin thread will be pulled to the top, evenly on both sides of the zigzag.
If either is to tight then the reverse is true.
There for, no matter what you do the stitches are not even on each of the zigzag on the back side.
The machine is out of adjustment. Once it is corrected you can play with the tension. Until then you are spinning your wheels.