There are a couple of reasons your pancakes are not cooking in the middle. Your pan is too hot or your batter is too thick. If the pancakes begin to brown excessively (or the butter is burning), make sure to turn down the heat. Otherwise, try adding a bit more milk to thin out the batter. You can also turn the heat off the burner, then place a lid on top of the pan to help it "steam" and cook inside.