required field validators not firing when using ajax controls

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="ReviewAdd.aspx.cs" Inherits="TailspinSpyworks.ReviewAdd" %><asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server"></asp:Content><%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit.HTMLEditor" TagPrefix="cc1" %><%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %><asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server"><asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager><div class="ContentHead">Add Review - <asp:label id="ModelName" runat="server" /></div><div style="padding: 20px; border-style:solid; border-width: 1px"><span class="NormalBold">Name</span><br /><asp:TextBox id="Name" runat="server" Width="400px" /><br /><asp:RequiredFieldValidator runat="server" id="RequiredFieldValidator1" ControlToValidate="Name" Display="Dynamic" 
                                    CssClass="ValidationError" ErrorMessage="'Name' must not be left blank."  /><br /><span class="NormalBold">Email</span><br /><asp:TextBox id="Email" runat="server" Width="400px" /><br /><asp:RequiredFieldValidator runat="server" id="RequiredFieldValidator2" ControlToValidate="Email" Display="Dynamic" 
                                    CssClass="ValidationError" ErrorMessage="'Email' must not be left blank." /><br /><hr /><br /><span class="NormalBold">Rating</span><br /><br /><asp:RadioButtonList ID="Rating" runat="server"><asp:ListItem value="5" selected="True" Text='<img src="Styles/Images/reviewrating5.gif" alt=""> (Five Stars) '  /><asp:ListItem value="4" selected="True" Text='<img src="Styles/Images/reviewrating4.gif" alt=""> (Four Stars) '  /><asp:ListItem value="3" selected="True" Text='<img src="Styles/Images/reviewrating3.gif" alt=""> (Three Stars) '  /><asp:ListItem value="2" selected="True" Text='<img src="Styles/Images/reviewrating2.gif" alt=""> (Two Stars) '  /><asp:ListItem value="1" selected="True" Text='<img src="Styles/Images/reviewrating1.gif" alt=""> (One Stars) '  /></asp:RadioButtonList><br /><hr /><br /><span class="NormalBold">Comments</span><br /><cc1:Editor ID="UserComment" runat="server" /><asp:RequiredFieldValidator runat="server" id="RequiredFieldValidator3" ControlToValidate="UserComment" Display="Dynamic" 
                                    CssClass="ValidationError" ErrorMessage="Please enter your comment." /><br /><br /><asp:ImageButton ImageURL="Styles/Images/submit.gif" runat="server" CausesValidation="true" id="ReviewAddBtn" onclick="ReviewAddBtn_Click" /><br /><br /><br /></div></asp:Content>


using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Diagnostics;
using TailspinSpyworks.Data_Access;

namespace TailspinSpyworks
{
    public partial class ReviewAdd : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!String.IsNullOrEmpty(Request.QueryString["ProductID"]))
            {
                int  productID = 0;
                Int32.TryParse(Request["productID"].ToString(), out productID);
                using (CommerceEntities db = new CommerceEntities())
                {
                    try
                    {
                        var thisProduct = (from p in db.Products where p.ProductID 
                                               == productID select p).FirstOrDefault();
                        ModelName.Text = thisProduct.ModelName;
                    }
                    catch (Exception exp)
                    {
                        throw new Exception("ERROR: Unable to Add Product Review - " + exp.Message.ToString(), exp);
                    }
                }
            }
            else
            {
                Debug.Fail("ERROR : We should never get to ReviewAdd.aspx without a ProductId.");
                throw new Exception("ERROR : It is illegal to load ReviewAdd.aspx without setting a ProductId.");
            }

        }

        protected void ReviewAddBtn_Click(object sender, ImageClickEventArgs e)
        {
            try
            {
                if (Page.IsValid == true)
                {
                    // Obtain ProductID from Page State
                    int productID = Int32.Parse(Request["productID"]);

                    // Obtain Rating number of RadioButtonList
                    int rating = Int32.Parse(Rating.SelectedItem.Value);

                    // Add Review to ReviewsDB.  HtmlEncode before entry
                    using (CommerceEntities db = new CommerceEntities())
                    {
                        try
                        {
                            ShoppingCart cartadd = new ShoppingCart();
                            Review newReview = new Review()
                            {
                                ProductID = productID,
                                Rating = rating,
                                CustomerName = HttpUtility.HtmlEncode(Name.Text),
                                CustomerEmail = HttpUtility.HtmlEncode(Email.Text),
                                Comments = HttpUtility.HtmlEncode(UserComment.Content)
                            };
                            db.Reviews.AddObject(newReview);
                            db.SaveChanges();
                        }
                        catch (Exception exp)
                        {
                            throw new Exception("ERROR: Unable to Update Cart Item - " + exp.Message.ToString(), exp);
                        }
                    }
                    Response.Redirect("ProductDetails.aspx?ProductID=" + productID);
                }
                Response.Redirect("ProductList.aspx");
            }
            catch (Exception ex)
            {
            }
        }

        protected void LikeRating_Changed(object sender, AjaxControlToolkit.RatingEventArgs e)
        {

        }
    }
}

The validation controls not working in this page.Not able to configure what the problem is