make sure your dropdownlist tag has the attribute
AutoPostBack="true"
and page load method as below..
<asp:dropdownlist id="ddlLocation" autopostback="true" runat="server" onselectedindexchanged="ddlLocation_SelectedIndexChanged" xmlns:asp="#unknown"></asp:dropdownlist>
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
}
}</pre>