GridView 选择框里的Checked值都是false

aspx里的：
<asp:GridView ID="GridView1" runat="server" Width="100%" CellPadding="0" ForeColor="#333333"
                        AutoGenerateColumns="False" AllowPaging="true" PageSize="12" BorderColor="Silver"
                        BorderStyle="Solid" BorderWidth="1px" OnRowDataBound="GridView1_RowDataBound"
                        EmptyDataText="没有数据记录！！" DataKeyNames="id" AllowSorting="True">
                        <Columns>
                            <asp:TemplateField HeaderText="选择">
                                <HeaderTemplate>
                                    <asp:CheckBox ID="chkAll" runat="server" ToolTip="全选" onclick="chkAll_Click(this)"
                                        Text="分店名称" />
                                </HeaderTemplate>
                                <ItemTemplate>
                                    <asp:CheckBox ID="chkItem" runat="server" ToolTip="选择" Text='<%# Eval("d_name")%>' onclick="chkItem_Click(this)" />
                                </ItemTemplate>
                                </asp:TemplateField>
                                <asp:BoundField HeaderText="状态" DataField="zt" />
                                <asp:BoundField HeaderText="单号" DataField="yewu_id" />
                                <asp:BoundField HeaderText="地址" DataField="addr" />
                                <asp:BoundField HeaderText="希望日期" DataField="xw_date" DataFormatString="{0:yyyy-MM-dd}" />
                                <asp:BoundField HeaderText="机型" DataField="ji_x" />
                                <asp:BoundField HeaderText="故障描述" DataField="g_z" />
                                
                        </Columns>
                        <RowStyle BackColor="#F7F6F3" ForeColor="#284775" HorizontalAlign="Left" />
                        <EditRowStyle BackColor="#999999" />
                        <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
                        <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
                        <AlternatingRowStyle BackColor="#A4D3EE" ForeColor="#333333" />
                        <PagerSettings Visible="False" />
                        <HeaderStyle Font-Bold="False" Font-Italic="False" Font-Size="12px" Height="25px"
                            BackColor="#8DB6CD" HorizontalAlign="Center" />
                    </asp:GridView>CS里的
protected void Button2_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < GridView1.Rows.Count; i++)
        {
            System.Web.UI.WebControls.CheckBox chk = (System.Web.UI.WebControls.CheckBox)GridView1.Rows[i].FindControl("chkItem");
            if (chk.Checked == true)
            {
                int ddsID = Convert.ToInt32(GridView1.DataKeys[i].Value.ToString());
            }
        }
        //Response.Redirect(url);
    }
chk 取出来的checked值都是false， 我选择了,checked的值还是false
asp:CheckBox这个是错在哪里？
因为我这个页面还有用导出EXCEL