It can be a " little thing" that can cause tension nightmares.........needle might not be perfectly centered, your bobbin may not be in the bobbin case correctly...mine must turn counterclockwise or else! Bobbin might be too loose...when you hold the thread and drop the bobbin, does it just slide down very slowly, drop like a rock or not move at all......on my machine, it should just drop slowly.........that little screw on side controls that...is the case perfectly round....dropped case can get a dent and throw the whole process off......each machine has its basic things that allow it to make the perfect stitch....it's up to the owner to find and keep in check those fine points.......