Since there are no eggs to provide leavening in vegan baking, cakes rely entirely on chemical leaveners, such as baking powder and soda. If you mismeasure these critical ingredients, there will be dire consequences, so be diligent and stick to the recipe as written! Tweaking flavors and playing around to put your own spin on things is encouraged, but altering the basic structure is not recommended.
Also, be certain to check that both baking powder and soda are in good working condition. Those little boxes tend to stick around forever, and if you don’t do a whole lot of baking, chance are they’ve gone bad and lost their leavening ability. To test the efficacy of baking powder, place 1 teaspoon into a small dish, and mix in ½ teaspoon water. For baking soda, you want to combine 1 teaspoon with ½ teaspoon of vinegar. In both cases, they should bubble up right away, or else it’s time to replace them.