Grid disappear after postback from button search

hi

I am searching members through a textbox and then click a button Go. 

Before adding Dropdownlist in the grid, i was able to search for member and it display the result on the grid if found.

After adding a dropdownlist in the GridTemplateColumn, the grid disappear and display only dropdownlist.

 Have I done something wrong? Thanks a lot

<%@ Page Title="" Language="C#" MasterPageFile="~/crm/layout.master" AutoEventWireup="true" CodeFile="registration.aspx.cs" Inherits="registration" %>

<%@ Register src="../usercontrol/title.ascx" tagname="course" tagprefix="uc1" %>

<%@ Register assembly="Telerik.Web.UI" namespace="Telerik.Web.UI" tagprefix="telerik" %>

<asp:Content ID="Content1" ContentPlaceHolderID="Left" Runat="Server">
    <uc1:Title ID="Title1" runat="server" />
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="Middle" Runat="Server">
<div class="container">
         <h4>REGISTRATION</h4>
        <hr />
    <div>
       <div  class="col-xs-4">
            <div class="input-group">
                  <span class="input-group-btn" >
                      <asp:TextBox ID="txtSearch" runat="server" CssClass="form-control" placeholder="Search for..." />
                      <asp:Button ID="btnserach" cssclass="btn btn-default" runat="server" Text="Go" OnClick="btnserach_Click" />&nbsp;
                  </span>
            </div>
        </div> 
    </div> 
</div>
<div style="margin-left:5px;margin-top:15px;margin-right:5px">
<div>
    
        <telerik:RadGrid ID="RadGrid1" runat="server" CellSpacing="0" Skin="Metro"  GridLines="None" OnNeedDataSource="RadGrid1_NeedDataSource">
            <MasterTableView AutoGenerateColumns="False" DataKeyNames="MemberID" >
                <Columns>
                       <telerik:GridBoundColumn DataField="MemberID" FilterControlAltText="Filter MemberID column" HeaderText="Member ID" SortExpression="MemberID" UniqueName="MemberID">
                        <ColumnValidationSettings>
                            <ModelErrorMessage Text="" />
                        </ColumnValidationSettings>
                    </telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="Gender" Display="false" FilterControlAltText="Filter Gender column" HeaderText="Gender" SortExpression="Gender" UniqueName="Gender">
                        <ColumnValidationSettings>
                            <ModelErrorMessage Text="" />
                        </ColumnValidationSettings>
                    </telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="FullName" FilterControlAltText="Filter FullName column" HeaderText="Name" SortExpression="Salutation" UniqueName="FullName">
                        <ColumnValidationSettings>
                            <ModelErrorMessage Text="" />
                        </ColumnValidationSettings>
                    </telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="Email" FilterControlAltText="Filter Email column" HeaderText="Email" SortExpression="Email" UniqueName="Email">
                        <ColumnValidationSettings>
                            <ModelErrorMessage Text="" />
                        </ColumnValidationSettings>
                    </telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="DOB" DataType="System.DateTime" DataFormatString="{0:yyyy/MM/dd}"  FilterControlAltText="Filter DOB column" HeaderText="DOB" SortExpression="DOB" UniqueName="DOB">
                        <ColumnValidationSettings>
                            <ModelErrorMessage Text="" />
                        </ColumnValidationSettings>
                    </telerik:GridBoundColumn>
                    <telerik:GridTemplateColumn UniqueName="Uname" HeaderText="Title">
                        <ItemTemplate>
                            <telerik:RadDropDownList RenderMode="Lightweight" runat="server" ID="CategoryIDDropDown" DataValueField="TitleID"
                                DataTextField="Title" DataSourceID="SqlDataSource1">
                            </telerik:RadDropDownList>
                        </ItemTemplate>
                    </telerik:GridTemplateColumn>
                    <telerik:GridButtonColumn CommandName="attendance" ConfirmText="Are you sure?" 
                        FilterControlAltText="Filter column column" HeaderText="Confirm Attendance" Text="Confirm Attendance" 
                        UniqueName="attendancecolumn" ItemStyle-ForeColor="#333333">
                        <ItemStyle ForeColor="#333333"></ItemStyle>
                    </telerik:GridButtonColumn>                
                </Columns>
            </MasterTableView>
        </telerik:RadGrid>

</div>
    <div>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="Title_SelectDropdown" SelectCommandType="StoredProcedure">
        </asp:SqlDataSource>
     </div>
</div>
</asp:Content>



using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class registration : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {

            Session["mark"] = " ";
            RadGrid1.Rebind();
        }
    }
    protected void btnserach_Click(object sender, EventArgs e)
    {
        Session["mark"] = txtSearch.Text;
        RadGrid1.Rebind();
    }
    protected void RadGrid1_NeedDataSource(object sender, Telerik.Web.UI.GridNeedDataSourceEventArgs e)
    {
        RadGrid1.DataSource = Memberlib.Member_SelectMark(Session["mark"].ToString());
    }
}