I can't remember who was having a problem with their thread bunching up on the bottom side of their sewing....BUT as I was trying to finish something last night, I had the same problem. I kept changing the tension and even took the bobbin mechanism out and put it all back together. I said words that would have caused my DD to start a prayer vigil for me if she had heard. :roll:
Finally I realized the bobbin I was using was thicker than the ones I usually use. It was the first I had used from a new pkg. and I didn't realize they were not the same size. When I changed back to the flatter one, everything was fine. You might check that.