It is important to keep in mind that over-long cooking after the sauce reaches its desired thickness can cause the starch granules to lose their ability to hold water and your sauce to "thin out." Luckily you can re-thicken your sauce by tempering in a bit more starch. You should also keep in mind that sauces thickened with starch will thicken even more once they are cooled down. This is because the water and starch molecules move with less and less energy and the starch eventually set into a stable structure with water trapped in between. To achieve the desired consistency of the sauce, you can make the sauce thinner at the stove than they are meant to be at the table, use a little less thickener, or gentlely reheating the sauce.