Setting tension on a longarm machine can be tricky! Renae Haddadin shares an exercise that will help you learn what changes cause what effect in your stitch. A Towa tension gauge is very helpful in finding the best tension for the bobbin. Once that is achieved it isn’t hard to adjust the top tension. TOWA guages are available here
Another great tutorial is from Jamie Wallen using his A1 longarm machine.
Hope both of these will assist you in becoming comfortable in handling the tension adjustments on your machine with different threads.
The main thing to remember always is:
What have you done since your machine was stitching fine?
- changed needle
- changed thread
- changed quilt batting/sandwich
- changed bobbin
then work backwards checking these until you narrow down the issue.