There are a few potential reasons why your cornbread might not be rising in the middle. One possibility is that you're using too much flour. When you add too much flour to a recipe, it can make the bread dense and heavy, which prevents it from rising. Another possibility is that your baking powder is old or expired. Baking powder is what helps breads and cakes to rise, so if it's not fresh, your cornbread won't rise either. Finally, it could be that your oven isn't hot enough. Bread needs to be baked at a high temperature in order to rise properly, so if your oven isn't hot enough, the bread won't rise.
If you're not sure what the problem is, try making a smaller batch of cornbread using less flour and fresher baking powder. If that doesn't work, make sure your oven is preheated to the correct temperature before baking the cornbread. With a little troubleshooting, you should be able to get perfect results!