why my event recevier is firing when "ItemAdded" but not when "ItemUpdated"

I have created an event receiver using Visual studio professional 2012. when i first created i specify that the event receiver will fire when "item was created". and i provided mu business login as follow:-

public class EventReceiver1 : SPItemEventReceiver
    {
        /// <summary>
        /// An item was added.
        /// </summary>
        ///
        public override void ItemAdded(SPItemEventProperties properties)
        {
            base.ItemAdded(properties);
//code goes here
        }
    }

now i need this event receiver to also fire when an item is updated , so i provide the following method:-

public class EventReceiver1 : SPItemEventReceiver
    {
        /// <summary>
        /// An item was added.
        /// </summary>
        ///
        public override void ItemUpdated(SPItemEventProperties properties)
        {
            base.ItemUpdated(properties);
//code goes here

then i deploy the event receiver.but now when i add an item the ItemAdded method will get fired. but when i update an item the ItemUpdated method will not get fired? so can anyone adivce on this please?