I have made this recipe: http://www.sixsistersstuff.com/2015/01/chocolate-zucchini-sheet-cake.html , twice, the first time it turned out absolutely wonderful, and the second cake out rubbery (but still tasty). I'm about to make it a third time and was wondering what I might have done wrong the second time around? Too much zucchini or not enough? Or maybe too much or too little flour? What would make a cake like this rubbery?