为什么Ajax只适用于最新的帖子

我想使用Ajax在不刷新的情况下调用PHP页面。

这是我的代码

<?php 

global $db;
$cmd = "SELECT * FROM posts order by date desc";
$result = mysqli_query($db,$cmd);
while ($row = mysqli_fetch_assoc($result)){
    $post_txt = $row['text'];
    $post_image = $row['image'];
    $postid = $row['postid'];
    $date = $row['date'];
    $userid = $row['userid'];
    $new_date=time_elapsed_string($date);

    ?>

    <?php
    global $db;
    $sql = "SELECT * FROM user WHERE id = '$userid'";
    $result2 = mysqli_query($db,$sql);
    while($row2 = mysqli_fetch_assoc($result2)){
        $acountname = $row2['name'];
        $acountlast_name = $row2['lastname'];
    } 
    ?>

<?php

echo"
<div id='postShare'>
    <div class='col-xs-7' id='listPost'>
                <div class='list-group'>    
            <input type='hidden' value='$userid' id='user_id_input_hidden'>     
            <li class=list-group-item text-center'><span class='pull-right'><p><a href='profile.php' id='prof_btn'>$acountname  $acountlast_name</a></p></span>&nbsp;
            <span class='pull-left'><p>$new_date</p></span>
            <hr>
            <center><h5 id='show_post_title'>$post_txt</h5></center> <hr>";?>

我想用Ajax加载profile.php：

$(document).ready(function(){
    $('#prof_btn').click(function(){
        var userid = $('#user_id_input_hidden').val();
        var dataString = 'userid=' + userid;
        $.ajax({
          url:'profile.php',
          type:'POST',
          data:dataString,
          success:function(prof){
            $('html').empty();
            $('html').append(prof);
          }
        });
        return false;
    });
});

但它只适用于echo中的最新帖子，不适用于任何帖子。怎么了？