I had a machine that zig'd but didn't zag. I ended up having to re-time it's bobbin hook.Since it was an old machine that i essentially had gotten for free i googled/youtubed it then fiddled with it one afternoon. Before i even touched that gear i used a piece of paper to see if it was only stitching one line or going both way. Turns out when it did try to zigzag it wasn't picking up the zag thread and was just being pulled through the zig stitch. See i had both parallel needle holes but only a straight stitch was being formed.