Make sure you're using medium heat or even medium high heat. Cornstarch must reach about 203°F in order to gelatinize. If your heat isn't high enough, it might not be reaching the temperature it needs to thicken the mixture. You can always double check the temperature of the ice cream base with an instant read thermometer. Also, make sure the cornstarch isn't clumped up. This would impede the mixture from evenly thickening. To avoid this, add the cornstarch in batches while continuously whisking.