It's possible your baking soda is expired, or you added too much of it. You may also need to add an extra 1/4 teaspoon of baking soda if you use a flax egg instead of egg replacer (as I explain below). Also, your cake won't rise quite as much if you're using a 9x5 inch loaf pan instead of an 8½ in. x 4½ in. loaf pan.