I would go through the basic points, check thread route, take out the bobbin case, double check for correct postion and it clicks correctly in place. You could always oil the edge the bobbin case rests on. Check if the needle goes all the way in, sometimes dirt and grime has accumulated behind the clamp. Make sure the needle tip is sharp and fine. I usually only adjust upper tension for double needle, but I often adjust bobbin tension for thicker thread, like going from sew-all weight to top stitch. Bobbin in the correct way! Check for lint or piece of thread in the top tensioner.
If the stitching is fine on two layers of quilting cotton the trouble might be else where. Fine slippery jersey with viscose and lycra can act up, personally I have found lessening presserfoot tension has helped a lot. Sometimes a microtex neelde or ball point needle turn out to be a must. If it's a firmer all cotton knit it tends to be less fuzzy. Matching needle and thread tickness is important too, it needs to be within reason. Sew all Güterman polyester is very easy to get a nice stitch with, and much the same for good quality mercerized cotton. I use all kinds of thread, but when I am trouble shooting I aim for the most dependable thread I have.