This particular problem could be because of many reasons. The most possible is that the batter is not thoroughly mixed. Check with your recipe and try to experiment to ensure that the ingredients are thoroughly mixed. It is also important that the mixture is properly aerated for the correct duration. The problem could also be because of the inconsistency of the batter. It is also best to check the temperature of the batter and the cooking temperature to ensure that they are not too low preventing the batter to rise.