This is likely because the pan is not hot enough and the cornbread batter is wet and sticks to the bottom of the pan before it can cook. You can avoid this by doing two things: when you preheat the oven, pop cast iron skillet in there to warm up. Then, before you add batter, smear a tablespoon of vegetable oil and evenly coat the skillet. Be careful to use an oven mitt when you pull your cast iron skillet out!