Cakes can come out dense because of the following reasons:
1. Not enough raising agents. Its the leavening that helps in making light cakes and if there is insufficient amount, it could lead to making the cakes denser. Try adding a bit more leavening in the recipe.
2. Over beating the batter. Gently mixing and folding the batter helps to incorporate air into the batter making it light and fluffy when baked. Overbeating removes the air pockets making the end result drier and dense.