If you've simmered your syrup for 10-15 minutes and it still hasn't thickened, you can stir in a cornstarch slurry to help. Start with 1-2 teaspoons of cornstarch mixed with 2-4 teaspoons of water and stir it into the syrup while simmering. Continue to cook until a thicker syrup is achieved. Keep in mind that the syrup will continue to thicken as it cools so don't make it too thick!