When cooking teriyaki sauce, you have to reduce it more than think; the goal is to remove water from the mixture and therefore, concentrate the sweet, salty, umami flavors. You also need to use a thickener, such as cornstarch or arrowroot starch. You want the sauce is be almost syrupy with a viscosity somewhere between agave nectar and molasses. At that point, pull it from the heat to avoid the sugars burning.