It can be tricky to adjust the thread tension, when you are first getting to know your industrial machine. Here are the variables that will affect your stitching:
- Thread fibre (nylon, polyester, linen, etc.)
- Thread size (in Canada industrial cones of nylon thread used for leather/upholstery are numbered ie # 33, 46, 69, etc.)
- Needle size
- Needle point (leather point, regular point, ball point)
- Upper tension
- Lower tension
- Type and texture of the leather you are sewing
It looks to me like your upper tension is too tight, and your lower tension may be slightly loose. An easy way to check this is by using different colors of thread. Ideally the stitches on both sides should look identical with no thread peeking out from below, and should be nicely embedded in the leather without seeming too tight/hard to the touch. You may have to play around with adjusting both the upper and lower tension to work together as they should. Check your needle that the point is not hooked. Check that the needle is inserted correctly (long groove faces left). Check that there is no debris in the upper or lower tension spring. Check that your upper thread is feeding smoothly. Make sure your machine is well lubricated. I always do a test run on whatever I am sewing. Thin garment leather needs a different thread and tension than hard belt leather, for example.
And definitely keep the needle in more than 1/2 way when you are pivoting the corner.