Page not refreshing partially
<p>I am using ASP.net 3.5 and Ajax toolkit is 2.0. Following is my code and my page is not refreshing partially. Is there any way to debug where is the issue?</p>

<pre><code>&lt;form id="form1" runat="server"&gt;
    &lt;div&gt;
        &lt;asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="true"&gt;
        &lt;/asp:ScriptManager&gt;
        &lt;asp:DropDownList ID="ddl_country" runat="server" Style="width: 100%;" OnSelectedIndexChanged="ddl_country_SelectedIndexChanged"
            AutoPostBack="true"&gt;
        &lt;/asp:DropDownList&gt;
        &lt;asp:UpdatePanel ID="UpdatePanel1" runat="server"&gt;
            &lt;ContentTemplate&gt;
                &lt;div id="div_Port_ddl" runat="server" style="width: 100%;"&gt;
                    &lt;asp:DropDownList ID="ddl_port" runat="server" AutoPostBack="True" Width="100%" onchange="SetPortId(this);"
                        OnSelectedIndexChanged="ddl_port_SelectedIndexChanged"&gt;
                    &lt;/asp:DropDownList&gt;
                &lt;/div&gt;
            &lt;/ContentTemplate&gt;
            &lt;Triggers&gt;
                &lt;asp:AsyncPostBackTrigger ControlID="ddl_country" EventName="SelectedIndexChanged" /&gt;
            &lt;/Triggers&gt;
        &lt;/asp:UpdatePanel&gt;
    &lt;/div&gt;
    &lt;/form&gt;
</code></pre>
