Few possible reasons for this, especially if you are using a store bought mix could be:
1. You over mixed the batter. Too much mixing can make it dry.
2. You baked it longer than required and/or your Oven temperature is off.

If, inspite of precautions, if the end result is dry, try any of the following suggestions.
- adding an additional egg along with a tablespoon of honey.
- adding a tablespoon of sour cream
- adding about 1/3 cup of frozen corn
- replacing some of the liquid ingredients with creamed corn.

Try this recipe for a skillet Cheddar and Jalapeno Cornbread that's moist and a great addition to your family dinner table.