The most likely cause of your cornbread sticking in a cast iron skillet is that the skillet wasn't properly pre-heated and oiled. Or the cast iron has not been properly "seasoned" yet. To prevent your cornbread from sticking, make sure to preheat your cast iron skillet before adding any ingredients. It's important to let it come up to temperature before adding the batter. Once the skillet is heated, add a thin layer of Crisco and swirl it around to evenly coat the pan. This will help ensure that your cornbread doesn't stick when baking.