Overbaking could also be the culprit. The thin layer of cake batter in the typical 15- by 10- by 1-inch baking pan takes only about 10 to 15 minutes to bake. Check your cake after 8 or 9 minutes and watch closely until it tests done. It should be lightly browned, and the top should spring back when lightly touched.
Remove the cake from the pan onto a towel as soon as you take it from the oven. Roll up the cake in the towel and cool on a wire rack.