A common culprit of a dense, tough, not-at-all-fluffy cake is overmixing the batter. This cake should be mixed until thoroughly combined, but once it’s there, stop.  Another reason that your cake may not be fluffy is if you used expired (or just months-old) baking soda. It’s good to buy baking soda in the tiny boxes so that you can use up most of one in a short time. Once it’s open, it’s only good for six months. If in doubt, toss it and buy a new box (and label the next one so you know when to replace it).