If your sauce isn't thickening, it could be due to a couple of factors. First, make sure you're using the right amount of cornstarch and water for the slurry. Second, the cornstarch slurry needs to be heated to thicken. So, if the sauce is not hot enough, the thickening reaction may not occur. If the sauce still isn't thickening after these steps, you can prepare another batch of slurry, gradually add it to the sauce, and ensure it is heated while continuously stirring.