There are a few reasons your caramel sauce can become grainy. 1. The temperature is off. - Make sure that your stove is reading accurately and your caramel isn't cooking on too high of a heat. 2. You caramel cooked too long. - If you let your caramel cook too long on the heat, the sugar can crystalize. 3. It wasn't stored properly - Caramel sauce that has been stored at room temperature for too long can crystalize. Make sure that you are properly storing your caramel sauce in the fridge when you are not using it.