I got the thread on the bobbin, got the needle and bobbin threaded, plugged in, turned on, fabric pinned to stay where I need it. Fabric under the foot, start to sew, get a few stitches and back-stitch.... start to sew forward to seal a wee rip in a shirt and the needle stops moving, machine strains. I use the hand wheel and move the needle, get it loose, adjust as needed to start sewing again... jams again. Freezes up, won't move needle, machine strains. Raise the needle with the handwheel, raise the foot, remove fabric and there's a GIANT tangle and knot of thread on the bottom side of the fabric. I can pick it out and clean it all up but.... what am I doing wrong?