Caramel sauce can become gritty or grainy if the sugar crystallizes. This can happen for a number of reasons including not cooking the mixture long enough, or cooling the caramel sauce too quickly. Thankfully, it's really easy to fix grainy caramel. Place the caramel sauce and a little bit of water in a saucepan over medium heat. Bring it to a boil while stirring until the mixture is smooth. It should only take a couple of minutes.