I just completed a quilt that looked fine then I put on a practice sandwich to prepare for a class and even though I am using the exact same thread top and bottom, same batting and same needle that I used for that quilt, my stitches on the top of the practice quilt appear to be slightly diagonal. I am using a red thread on top and a medium blue on the bobbin of the same weight so I do see a slight blue dot on the top when the stitches form but I have to look for it. The back looks fine-- I've rethreaded, cleaned the bobbin case, bobbin area, oiled, turned the needle to 5:30 and then 6:30, loosened the sandwich on the frame, tightened it, increased tension and decreased tension top and bottom with no change. I have been at this for a couple of years now and had the problem once before but can't recall what actually fixed it. Is this an optical illusion, a timing issue, or something else?