If your rice pudding isn't thickening, try these two fixes: Cook It Longer: Give the pudding a bit more time on low heat. This helps the rice absorb more liquid and release starch, which thickens the mixture. Cornstarch Slurry: Mix a tablespoon of cornstarch with a little cold milk or water to form a slurry. Stir this into your pudding and cook for a few more minutes, stirring constantly. The cornstarch will help thicken it up.