It doesn't thicken enough when you don't add cornstarch or the amount of cornstarch might not be enough. We normally use a slurry made of 2 tablespoons of cornstarch and 1/2 cup milk to thicken the pudding. If your pudding is still too runny and doesn't look creamy at all (maybe because you use skimmed milk or plant-based milk), whisk extra 1 tablespoon of cornstarch and 1/4 cup of milk in a small bowl and pour it into the pudding. Stir constantly and cook for extra 5 minutes. This will help the runny rice pudding thicken. Also, keep in mind that it will thicken more as it cools down.