Postback to my method DropDownList2_SelectedIndexChanged is not being executed even though Autopostback is set to true

 
I have two drop down listboxes on a web form. When someone selects a value from the first listbox,
the second listbox is then loaded based on the value chosen in the first listbox. This works as it is suppose
to.
My problem is that when a value in the second drop down listbox is selected, the code behind method for the
second drop down listbox is never executed --> DropDownList2_SelectedIndexChanged(...)
Both drop down listboxes have Autopostback set to true so I don't know why the method DropDownList2_SelectedIndexChanged
is not being executed?
Why is that? Can someone help me out please? I need to determine why my method DropDownList2_SelectedIndexChanged is not
being executed?
 
<td>
    <asp:DropDownList ID="DropDownList1" runat="server"
        DataSourceID="SqlDataSource2" DataTextField=" ...