This is the key succes of making a moist muffin, stir the batter using a spatula or wooden spoon gently, make sure not to overmixed it. Give about 10 strokes just enough. If there are still have some dry spots, just leave it as it be, or you could just wet it by stir only at that spot, don't over do it. Lumpy batter is just perfect rather than overmixed it