AJAX function is not working by scroll

Using AJAX for endless scroll. Content loads only first time, but does't load by scroll.

What is wrong?

jQuery:

function loadFeed() {
    $.ajax({
        url: 'loadmore.php',
        dataType: 'html',
        success: function (data) {
            $("#posts").append('<div class="havanagila"></div>');
            $('#posts').html(data);
        }
    });
}

loadFeed();
$(window).scroll(function () {
    var windowScroll = $(window).scrollTop();
    var windowHeight = $(window).height();
    var documentHeight = $(document).height();

    if ((windowScroll + windowHeight) == documentHeight) {
        loadFeed();
    }
});

loadmore.php:

<?php 
session_start();

if ( isset( $_SESSION['login'] ) ) {

    $login    = $_SESSION['login'];
    $id=$_SESSION['id'];

    $username="root";
    $password="root";
    $hostname = "localhost";
    $dbname= "kotik";


    function testdb_connect ($hostname, $username, $password){
        $dbh = new PDO("mysql:host=$hostname;dbname=kotik", $username, $password);
        return $dbh;
    }


    try {
        $dbh = testdb_connect ($hostname, $username, $password);

    } catch(PDOException $e) {
        echo $e->getMessage();
    }

}

?>

<?php                                                              

$title_select_query= $dbh -> prepare("SELECT title FROM books WHERE id = :id ORDER BY date DESC");
$title_select_query ->execute(array(':id' => $id));
$title_select_query_result = $title_select_query->fetchColumn(); 
echo($title_select_query_result);

$title_select_query_result = $title_select_query->fetchColumn(); 
echo($title_select_query_result);

$title_select_query_result = $title_select_query->fetchColumn(); 
echo($title_select_query_result);

$title_select_query_result = $title_select_query->fetchColumn(); 
echo($title_select_query_result);

$title_select_query_result = $title_select_query->fetchColumn(); 
echo($title_select_query_result);

?>