You can use a crochet hook to hook up the missing stitch and connect it to the previous stitch and the next stitch, or use a special wool sewing needle to sew it from the inside, so it looks seamless from the outside. Knitting a sweater is a delicate job, and each stitch is connected from top to bottom. A missing stitch will not come off temporarily, but over time, one stitch after another will fall off, forming a big hole, which not only affects the appearance, but also makes it unwearable. Therefore, if you find that a sweater is missing a stitch, you should sew it up in time.