Ajax request not working, why?

Question

With this script I can show a counter next to <i class="fas fa-bell mr-3"></i> when you have data in db marked as status = unread, and when I click on the icon <i class="fas fa-bell mr-3"></i> updates certain Row to status = read.

When I enter data into my table with the status = unread the $countNT is incremented, until there is working perfectly and this $countNT is incremented without updating the page.

When I click on <i class="fas fa-bell mr-3"></i> I send a POST request with Ajax to the file update.php, and I can update the Row that used to be status = unread now would be status = read and the $countNT Zera.

The problem is that the request with Ajax for the file update.php is only running once on the page, example: if I updated the page about 10s ago, and then there is a notification, I can update the table in real time without having to refresh the page, but if I had already called the file update.php with Ajax on that page and then comes another notification, it will not be possible to update my database, I can only update the database if I reload the page. Can someone explain to me why?

html:

<div>
    <ul class="navbar-nav textoPerfilDesk dropMenuHoverColor">
        <li class="nav-item dropdown pr-2 dropleft navbarItem ">
        <a class="nav-link dropdown-toggle-fk" href="#" id="navbarDropdownMenuLink" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
            <i class="fas fa-bell mr-3"></i>
        </a>
        <div class="dropdown-menu dropdown-menu-fk py-3" aria-labelledby="navbarDropdownMenuLink">
            <a class="dropdown-item dropMNitemNT" href="um-link">
            <span class="d-flex">
                <img class="imgNT" src="img/1.jpg">
                <span class="pl-2 pt-1">
                    titutlo
                </span>
            </span>
            </a>
        </div>
        </li>
    </ul>
    <span class="text-white divCountNT" id="datacount"></span>
  </div>

script:

<script>
  $(document).ready(function(){

    $("#datacount").load("select.php");

    setInterval(function(){
    $("#datacount").load('select.php')
    }, 1000);

  });

  $(document).on('click', '.fa-bell', function (){

    $.ajax({
       type: "POST",
       url: "update.php"
    }).done(function() {
      $("#datacount").load("select.php");
    });

  });
</script>

select.php:

<?php
require_once 'db.php';

if(!isset($_SESSION))session_start();

if(isset($_SESSION['user_id'])) {
  $user_id = $_SESSION['user_id'];
}

$status = 'unread';

    $sql = $conn->prepare("SELECT * FROM noti WHERE status = :status AND user_id = :user_id");
    $sql->bindParam(':user_id', $user_id, PDO::PARAM_INT);
    $sql->bindParam(':status', $status, PDO::PARAM_STR);
    $sql->execute();
    $countNT = $sql->rowCount();

    echo $countNT;

    $conn = null;

    ?>

update php.:

<?php
require_once 'db.php';

if(!isset($_SESSION))session_start();

if(isset($_SESSION['user_id'])) {
  $user_id = $_SESSION['user_id'];
}

$status = 'read';
$sql = $conn->prepare("UPDATE noti SET status = :status WHERE user_id = :user_id");
$sql->bindParam(':user_id', $user_id, PDO::PARAM_INT);
$sql->bindParam(':status', $status, PDO::PARAM_STR);
$sql->execute();
$countNT = $sql->rowCount();

echo $countNT;

$conn = null;
?>