If your peanut sauce is watery, it could be because you added too much liquid or didn't cook it long enough. To thicken the sauce, you can simmer it over low heat until it reduces and thickens, or you can add a cornstarch slurry (a mixture of cornstarch and water) to help thicken it. Another option is to use less liquid or more peanut butter in the recipe to achieve a thicker consistency.