There could be a few reasons for this. It's important to spread the batter evenly in the skillet so it cooks uniformly. Also, be sure you're cooking it over medium heat. Too high, and it may cook too quickly on the outside while staying raw on the inside; too low, and it may dry out before cooking through. Lastly, make sure your skillet is well-heated before you add the batter – this helps with even cooking.