This is likely due to one of two common baking mistakes. Either: 1. You overmixed the batter. Stop stirring when the muffin mixture is just combined; a few lumps are okay! Or, 2. You added too much flour. To measure flour accurately, spoon the flour out of the bag and directly into the measuring cup. Make a high mound on the top of the cup, then level off the measuring cup using the back of a knife.