The biggest culprit to pancakes that are not fluffy is usually overmixing. Having a few lumps in your batter is perfectly fine. Letting the batter rest a bit also helps. Some people say that beating your eggs separately is another contributor for fluffy pancakes but I am much too lazy! I always find that as long as the griddle is hot enough and the batter is not overmixed and has rested my pancakes are always perfect!