First of all, do not rush the process of homemade balsamic reduction by overboiling. It is better to take the time to do it properly for at least 15 minutes, stirring occasionally. You should also use a heavy saucepan for even heat dissipation.  Remember that this sweet and tangy balsamic glaze will thicken slightly as it cools so make sure you let it cool completely before you panic. If it still is not thick enough after it chills, add a half teaspoon of corn starch and simmer on medium heat until it covers the back of a spoon.