DropDownList items return null when posting to edit form

<asp:TemplateField HeaderText="表格/檔案狀況" SortExpression="m_formstatus">              
                <ItemTemplate>
                    <asp:Label ID="Label2" runat="server" Text='<%# Bind("m_formstatus") %>'></asp:Label>
                </ItemTemplate>                     
                      <EditItemTemplate>
                        <asp:SqlDataSource ID="DS_m_servicestatus" runat="server" 
                            ConnectionString="<%$ ConnectionStrings:ConnectionString %>" 
                            SelectCommand="SELECT * FROM [m_servicestatus]"></asp:SqlDataSource>
                        <asp:DropDownList ID="ddl_formstatus" runat="server" DataSourceID="DS_m_servicestatus" DataTextField="m_estatuscodedesc" 
                        DataValueField="m_estatuscodedesc" >
                        </asp:DropDownList>                       
                    </EditItemTemplate>
            </asp:TemplateField

<asp:SqlDataSource ID="SqlDataSource1"  runat="server" ConflictDetection="CompareAllValues" 
        ConnectionString="<%$ ConnectionStrings:ConnectionString %>" 
        OldValuesParameterFormatString="original_{0}" 
        SelectCommand="SELECT * FROM [m_memserviceform]"
        UpdateCommand="UPDATE [m_memserviceform] SET [m_formstatus] = @m_formstatus WHERE [m_memcode] = @m_hkidno AND [m_serviceform] = @original_m_serviceform">

        <UpdateParameters>
            <asp:Parameter Name="m_hkidno" Type="String" />
            <asp:Parameter Name="m_formstatus" Type="String" />
        </UpdateParameters>>

protected void GridView1_PreRender(object sender, EventArgs e)
   {
       GridView1.DataSourceID = "SqlDataSource1";
       GridView1.DataBind();

       if (GridView1.Rows.Count == 0)
       {
           // Clear out the DataSourceID, otherwise our DataBind below will error
           GridView1.DataSourceID = "";

           // Create a new DataTable that mimics our original DataSource
           DataTable DT = new DataTable();
           DT.Columns.Add("m_memcode");
           DT.Columns.Add("m_serviceseq");
           DT.Columns.Add("m_serviceform");
           DT.Columns.Add("m_serviceformdesc");

           // Create our new blank row
           DT.Rows.Add(DT.NewRow());
           GridView1.DataSource = DT;
           GridView1.DataBind();
       }
   }

   protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
   {
       var conString = ConfigurationManager.ConnectionStrings["ConnectionString"];
       string strConnString = conString.ConnectionString;
       SqlConnection conn = new SqlConnection();
       using (conn = new SqlConnection(strConnString))
       {
           GridView1.DataSourceID = "SqlDataSource1";
           GridView1.DataBind();

           SqlDataSource1.SelectCommand = "SELECT * FROM [m_memserviceform] where m_memcode ='" + Request.QueryString["id"] + "'";

           SqlDataSource1.UpdateParameters["m_hkidno"].DefaultValue = m_hkidno.Text;

           //SqlDataSource1.UpdateCommand = "UPDATE [m_memserviceform] SET [m_formstatus] = @m_formstatus WHERE [m_memcode] = '" + m_hkidno.Text + "' AND [m_serviceform] = @original_m_serviceform";
       }

   }