why ajax work in some part of the page and dosent in some parts

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ASP.NET

when i click enter to abb comment with the btnaddcomment button the page dosent reload and the ajax word but when i click the add_post_btn button ajax dosent work and the page relod .....
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<%@ Page Language="C#" MasterPageFile="~/teacher/teacherMasterPage.master" AutoEventWireup="true"
    CodeFile="allposts.aspx.cs" Inherits="teacher_allposts" Title="posts" %>

<%--<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit.HTMLEditor"
    TagPrefix="cc1" %>--%>

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
<style type="text/css" >

#centertd
{ text-align:left;

}
</style>
  

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    
        
           
               
         <asp:UpdatePanel ID="postsajax" runat="server" >
         
                
                <contenttemplate>
                
                
               <br /><br /><br />
                
               
        
            <asp:TextBox ID="add_post_txt" runat="server" Height="59px" Width="416px" 
                        OnTextChanged="add_post_txt_TextChanged" BorderColor="Maroon" 
                        TextMode="MultiLine">
            <br />
            <asp:FileUpload ID="FileUploadPost" runat="server" Visible="false" />
 
             <br />
             
             
            <asp:Button ID="add_post_btn" runat="server" Text="Post" OnClick="btnInsertPost"
                Width="83px" ForeColor="Black" BackColor="#CCCCCC" />
            <asp:LinkButton ID="LinkButton1" runat="server" OnClick="LinkButton1_Click" ForeColor="Black">Upload 
                    what u want
                    
       
           
          
            
       
       
            <br />
        
                    <asp:Label ID="Label_upload_file" runat="server">
                    <br />
                   
         
            
                <%-----------------parent datalist for posts---------------------------------------%>
                
                <asp:DataList ID="posts_datalist" runat="server" DataKeyField="id" OnItemDataBound="posts_datalist_ItemDataBound"
                    OnSelectedIndexChanged="posts_datalist_SelectedIndexChanged" OnItemCommand="posts_datalist_ItemCommand"
                    OnDeleteCommand="posts_datalist_DeleteCommand" Width="400" 
                        style="margin-right: 0px">
                    <itemtemplate>
                   
                        <table border="0" width="400px">
                        
                            <tr>
                                
                                 
                                
                                    <td width="74px">   
                                           <asp:Image ID="post_user_image" runat="server" ImageUrl='<%#"../images/thump"+DataBinder.Eval(Container.DataItem,"picture") %>' Width="74px" Height="61px"/>
                                          
                                   </td> 
                                   <td id="centertd" align="left">  
                                          <asp:Label ID="Label1" Font-Italic=true ForeColor="Maroon" runat="server" Text=""><%# DataBinder.Eval(Container.DataItem, "user_name")%>
                                   </td>
                                   <td align="right">
                                  
                                        <asp:Button ID="dtndelete" runat="server" Text="X" Height="16px" Width="16px" CommandName="Delete" Visible="false" 
                                        
                                        
                                        
                                        
                                            CommandArgument='<%#Eval("id") %>'  />
                                   </td>
                             
                                
                                
                          </tr>
                          <tr width="500">
                          <td colspan="1"></td>
                                <td colspan="2" width="500">
                              
                                
                                  <div style=" height:100%; text-align:left; margin:10px 0px; width:200">
                                  
                                  <a id="LinkButtontext" href='<%#"../files/post/"+Eval("post_upload") %>' visible="false"  runat="server">
                                  <img src="../images/text.png" width="50px" height="50px" hspace="10" vspace="3"   />                                 
                                                                  
                                  <a id="LinkButtonword" href='<%#"../files/post/"+Eval("post_upload") %>'
                                  visible="false"  runat="server">
                                  <img src="../images/word.png" width="50px" height="50px" hspace="10" vspace="3"  />
                                                                                                  
                                  <a id="LinkButtonexcel" href='<%#"../files/post/"+Eval("post_upload") %>'
                                  visible="false"  runat="server">
                                 <img src="../images/excel.png" width="50px" height="50px" hspace="10" vspace="3"  />
                                 
                                   <a id="photo" href='<%#"../images/"+Eval("post_upload") %>'
                                  visible="false"  runat="server">
                                 <img src='<%#"../images/thump"+Eval("post_upload") %>' hspace="10" vspace="3" align="baseline"/>
                                 
                                                                    
                                  
                              <span>    <asp:Label ID="post_content" runat="server" Width="200" style="word-wrap:break-word"    Text="" ForeColor="Black" ><%# DataBinder.Eval(Container.DataItem, "post_content")%></span>
                                  <br />
                                                                                       
                                     <asp:Label ID="Label2"  runat="server" ForeColor="Black"  Font-Size="X-Small" Text='<%#Eval("date") %>'>
                                  
                                  </div>
                                    
                                </td>
                                
                            </tr>
                         
                            <tr id="trcommet"  runat="server" visible="false" height="5px">
                            <td colspan="1"></td>
                                <td id="td_comment" colspan="2" bgcolor="silver" visible="false">
                                   <%-- -------------- datalist of comments----------------------------------------%>
                                    <asp:DataList ID="comments_datalist" runat="server" DataKeyField="id">
                                        <itemtemplate>
                                           
                                       <table bgcolor="silver" border="0">
                                             
                                                 <tr width="100px" bgcolor="silver">
                                                    <td valign="top">
                                                        <asp:Image ID="comments_user_image" runat="server" ImageUrl='<%#"../images/thump"+DataBinder.Eval(Container.DataItem,"picture") %>' Width="40px" Height="30px"/>
                                                    </td>
                                                    <td align="left">
                                                    <asp:Label ID="labusernamecomment" Font-Italic=true  ForeColor="Maroon" runat="server" Text=""><%# DataBinder.Eval(Container.DataItem, "user_name")%>
                                                    <asp:Label ID="comment_content" runat="server" Width="300" style="word-wrap:break-word" Text="" ForeColor="Black"><%#DataBinder.Eval(Container.DataItem, "comment_content")%>
                                                    </td>
                                                    </tr>
                                                
                                            </table>
                                        </itemtemplate>
                                    
                <%-- //----------------------------------------end of comment datalist--------------------------------------------//--%>
        
                                </td>
                              
                                
                            
                            <tr>
                            <td colspan="1"></td>
                                <td colspan="2">
                                
                                    <asp:Panel ID="Panel1" runat="server" DefaultButton="btnaddcomment">                                                     
                                    <asp:Button ID="btnaddcomment" runat="server"  Text="add" CommandName="btn_comment_add"
                                        CommandArgument='<%#Eval("id") %>' style="display:none" />
                                    <asp:TextBox ID="txtComment"  runat="server"  Width="400px" Height="20px" Visible="false" >
                                    
                                      
                                    
                                    
                                    
                                    <asp:LinkButton ID="comment_link" ForeColor="Maroon" Font-Size="X-Small" Font-Bold="true" runat="server" CommandName="linkcomment" CommandArgument='<%#Eval("id") %>'>Comment
                              
                                 
                                </td>
                            
                               
                                    
                                
                            </tr>
                        </table>
                        <hr color="black" />
                         
                    </itemtemplate>
                
                
                <asp:LinkButton ID="prv" runat="server" onclick="prv_Click"><<
                           
                <asp:LinkButton ID="next" runat="server" onclick="next_Click">>>
                
                    <asp:Label ID="lblError" runat="server"  Visible="false">   
                
        </contenttemplate>
        <triggers>
        <asp:PostBackTrigger ControlID="add_post_btn" />
        
        </triggers>

-----------------------------------------------------------------------------------when i click enter to abb comment with the btnaddcomment button the page dosent reload and the ajax word but when i click the add_post_btn button ajax dosent work and the page relod .....