Infinite scroll with jquery and php - php

Question

I use this function to create an infite scrolling on my page. The thing is it displays the default div's (onload) and one batch with divs when i scroll to the bottom. But when i scroll to the bottom once again when the batch is added it won't trigger the loadmore.php function. It says: no more posts to show
What am i doing wrong?
<div id="wrapper">
<div id="postswrapper">
<div class="postitem" id="4"><img src="http://ww.supair.fr/photos_cam1_fixe/image.jpg" /></div><div class="postitem" id="5"><img src="http://www.bormioonline.com/imagesntent/webcam/webcam06-320x240right.jpg" /></div>
</div>
<script type="text/javascript">
var loading = false;
$(window).scroll(function(){
var h = $('#postswrapper').height();
var st = $(window).scrollTop();
// the following tests to check if
// 1. the scrollTop is at least at 70% of the height of the div, and
// 2. another request to the ajax is not already running and
// 3. the height of the div is at least 500.
// You may have to tweak these values to work for you.
if(st >= 0.7*h && !loading && h > 500){
loading = true;
$('div#loadmoreajaxloader').show();
$.ajax({
url: "/test2/loadmore.php?lastid=" + $(".postitem:last").attr("id"),
success: function(html){
if(html){
$("#postswrapper").append(html);
$('div#loadmoreajaxloader').hide();
}else{
$('div#loadmoreajaxloader').html('<center>No more posts to show.</center>');
}
loading = false;
}
});
}
});