请教各位大侠（暂容小弟放首页一晚上）

页面采用微软的AJAX框架，当一开始进入页面，且立即点击按钮时能够正确跳转，但如果在选择列中勾选或者过了几秒种后再点击按钮时，按钮没有反应，也不执行Click事件。调了一下午也没弄出个原因来，还望大侠出手相救。

前台代码：

 1<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
 2    <base target="_self" />
 3    <div>
 4        <asp:Label ID="lblToBeParchased" runat="server" Text="【待采购订单】" SkinID="Title"></asp:Label>
 5        <br />
 6        <asp:Panel ID="Panel1" runat="server" Width="820px" Height="420px" BorderStyle="Outset" ScrollBars=Auto>
 7                    <asp:Label ID="lblCheckOrder" runat="server" Text="请选择订单:"></asp:Label>
 8                    
 9                   <asp:Panel ID="pnlToBeParchased" runat="server" Width="1000px" Height="400px"><asp:GridView ID="gvToBeParchased" runat="server" DataKeyNames="IAuto_id" OnRowDataBound="gvToBeParchased_RowDataBound"
                                    AutoGenerateColumns="False">
                                    <Columns>
                                        <asp:TemplateField ShowHeader="False">
                                            <ItemStyle Width="4px" />
                                            <HeaderStyle Width="4px" />
                                            <ItemTemplate>
                                                <asp:CheckBox ID="chkSelectBox" runat="server"></asp:CheckBox>
                                            </ItemTemplate>
                                        </asp:TemplateField>
                                        <asp:BoundField DataField="IAuto_id" HeaderText="自动编号" Visible="False" />
                                        <asp:BoundField HeaderText="订单号" DataField="or_code">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="产品名" DataField="p_name">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="规格" DataField="p_spec">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="交货期" DataField="oi_date" DataFormatString="{0:yyyy-MM-dd}"
                                            HtmlEncode="False">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="订单量" DataField="oi_num">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="玻管编号" DataField="p_tubecode">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="玻管量 " DataField="tub_num">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="瓶盖名" DataField="p_clocode">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="瓶盖总量" DataField="pur_clo_num">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="已采购量" DataField="tub_received">
                                        </asp:BoundField>
                                        <asp:BoundField HeaderText="已采购量" DataField="clo_received">
                                        </asp:BoundField>
                                    </Columns>
                                </asp:GridView>
                    </asp:Panel>
                </asp:Panel> 
        <asp:UpdatePanel ID="upnlToBeParchased" runat="server" ChildrenAsTriggers="False"
            UpdateMode="Conditional">
            <ContentTemplate>
                
                <table width="800">
                        <tbody>
                            <tr>
                                <td align="center">
                                    <table>
                                        <tbody>
                                            <tr>
                                                <td style="width: 318px" colspan="3">
                                                    <asp:ImageButton ID="ibtnNextStep" OnClick="ibtnNextStep_Click" runat="server" ImageUrl="~/Images/NextStep.gif">
                                                    </asp:ImageButton>
                                                   
                                                </td>
                                            </tr>
                                        </tbody>
                                    </table>
                                </td>
                            </tr>
                        </tbody>
                    </table>
            </ContentTemplate>
            <Triggers>
                <asp:PostBackTrigger ControlID="ibtnNextStep" />
            </Triggers>
        </asp:UpdatePanel>
    </div>
</asp:Content>

Click事件：

 1protected void ibtnNextStep_Click(object sender, ImageClickEventArgs e)
 2    {
 3        string str = "";
 4        for(int i=0;i<gvToBeParchased.Rows.Count;i++)
 5        {
 6            CheckBox chk = (CheckBox)gvToBeParchased.Rows[i].Cells[0].FindControl("chkSelectBox");
 7            if (chk.Checked)
 8            {
 9                str += gvToBeParchased.DataKeys[i]["IAuto_id"] + ",";
            }
        }
        if (str == "")
        {
            ScriptManager.RegisterClientScriptBlock(upnlToBeParchased, typeof (UpdatePanel), "okBack","alert( '请至少选择一条数据 ');", true);
            return;
        }
        if (str.Substring(str.Length - 1, 1) == ",")
        {
            str = str.Substring(0, str.Length - 1);
        }
        Session["orders"] = str;
        Response.Redirect("PurDetail.aspx?mode=add&type=" + ViewState["type"].ToString().Trim() + "&source=2");
    }