I have never had to touch the sub tension knob. There is a diagram on page 17 of the manual. I have my tension set about 3.5. Pressure foot regulator, this is in the manual. For FMQ, I have it all the way up at the top. The rest of the time it's on/near the middle dot. Make sure you have your thread between the tension discs and that there are no little bits of thread stuck there. Be sure your feed dog lever is all the way over. Move it back and forth a few times.