A soufflé can fall if the egg whites aren't beaten long enough or if they're beaten too long. They should reach firm peaks so make sure they go just beyond soft peaks and keep their shape when the beaters are lifted. If they're under-beaten, you'll see the peaks retreat back into the bowl instead of keeping their shape. Also, if the door of your oven is opened frequently during the baking process, the influx of cold air can also prevent your soufflé from setting properly.