Problem to event

Hi,
OnSelectedIndexChanged event below is not fired, after I've selected one value to DDL. Why?

    $("#Panel0").dialog({
        autoOpen: true,
        appendTo: "#main_form",
        autoResize: true,
        modal: true,
        height:580,
        width: 620,
        position:myPos,
        resizable: true,
        title: "快速尋找",
        open: function (event, ui) {
            $(".ui-widget-overlay").css({ "opacity": ".7", "background": "#9DADB3" });
        }
    });

    <div id="Panel0" align="center"  runat="server">
        <div align="center" runat="server" enableviewstate="true" style="width: 600px; height: 120px;background-color:#3d7c94">
            <br /><br />
            <asp:Label ID="Label7" runat="server" ForeColor="#0099ff" Font-Bold="true" Font-Size="XX-Large" Text="test" />
        </div>
        <asp:DropDownList ID="pop_country_ddl" OnSelectedIndexChanged="pop_country_ddl_value_changed" Width="190px" runat="server">
        </asp:DropDownList>

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