If the brown sugar syrup of your candied sweet potatoes is runny you can thicken it very easily. Mix 2 tablespoons of corn starch and 2 tablespoons of cold water in a small bowl. Pour the corn starch slurry into the brown sugar glaze and carefully stir. Try not to break up the pieces of sweet potatoes. Heat over medium high heat until thickened or return the dish to the oven and continue to cook until thickened.