Are you new to long arming? I found that when I have the quilt too tight on the frame (remember you should be able to grasp a finger poked up from underneath) that it causes poor tension and makes the back stitches look like they didn't "sink" in. I rarely touch my top tension anymore since I started checking my tension with each bobbin(my Towa gauge became my new BFF). I've found that some threads (particularly very fine) seem to wind tighter so I need to adjust the tension. Typically, I'll check the tension on that first line of stitching to attach the bat. if all is well with it, I'll go ahead, check underneath (I have a mirror to do that!) on the first part . If I'm having issues, I'll lay a piece of fabric on the excess to the edge of the quilt (on top of the bat & backing) and test there so my test is on the same fabrics/bat/frame tension. It took me at least a year of working on long arm to really feel confident on my tension adjustments (am I a slow learner?) LOL