NEVER cook your cornstarch that long.
You'll cause it to backfire and lose it's thickening properties.
I don't know if you did this but just in case:
First you must wisk the cornstarch with a liquid such as water, rice wine, stock, whatever. This is called a slurry and prevents lumping. The slurry should be the consistency of heavy cream
Add the slurry to simmering, not boiling liquid and then return it to JUST UNDER A BOIL.
Every chef in the world will tell you to bring your roux or cornstarch/arrowroot thickened sauce to a full boil to achieve its maximum thickening potential. This is not scientifically correct. Beyond 200-205 degrees (boiling is 212), the starch will begin to break down. Thus, bring it to almost a full boil, immediately reduce to a simmer, and stir gently. It only needs a few minutes to hit it's maximum thickening potential.
One tablespoon of cornstarch will thicken about 1 1/2 - 2 cups liquid. Measure accordingly based on the viscosity that you want.
Mark