The temperamental nature of soufflés makes them a notoriously difficult dessert to master. Egg whites beaten until they're light and fluffy are what makes this showcase dessert puff up with its characteristic top-hat-shaped cylinder when baked. Many a sit-com episode has centered on the soufflé gone wrong.
The ingredients you include in the soufflé, whether it's chocolate or lemon or some other sweet combination, have to be the right density. If the flavoring is too dense, it might weigh down the soufflé and prevent it from rising. If it's not dense enough, you might get a runny mess. All of the ingredients must be handled carefully and delicately so as not to lose the foamy lightness of the eggs that is crucial to the success of this dessert.
Then there's the cooking process -- if you undercook it, the soufflé will deflate. And if you overcook it, it could also deflate. Most importantly: do not open the oven while it bakes! Otherwise -- you guessed it -- it might deflate.