Web Forms :: AutoPostBack Does Not Work?

I am an asp.net newbie and encountered problem with postback. I have created a web form and put some web controls and set their AutoPostBack property to True.

Some of the controls are listbox, checkbox. However, the SelectedIndexChanged and OnCheckChanged do not fire automatically when an item is selected from the listbox for example or when the checkbox is checked. I noticed that the commands I put in the SelectedIndexChanged and OnCheckChanged is only fired when a button is clicked.

e.g. The code below that sets the text of a label is being fired only when a button is clicked even if the autopostback property of the listbox is set to TRUE.

protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
{
ListItem myItem = ListBox1.SelectedItem;
Label1.Text = myItem.Text;
}

Again, I would like the Label's text to change when an item is selected from the listbox and not only when a button is clicked.

What else could I be missing to make autopostback work?