One thing that people forget about is that when the presser foot is UP, the thread will slip in between the tension discs easily. But, when the presser foot is DOWN, the tension discs are locked and the thread won't go between the discs or sew correctly. Always thread your machine with the presser foot UP, then, when you lower the foot to sew, the discs snug the thread and the sewing will come out correctly....well, if everything else is done correctly.... like threading the needle from the correct direction, putting the bobbin in right, etc.