Originally Posted by LadyMischief

It could be a lot of factors.. did you open the oven while it was cooking? Not generally a good thing better to let the cake be until you are done. How long was that package on the shelf? The levening agent might not have been as active anymore, the humidity may have changed the consistancy of the flour in the mix and prevented it from rising as well, or you could have possibly beaten the mixture too hard, activating the gluten in the wheat and making it more elastic and solid and less fluffy and airy.