jquery滚动功能不起作用

I am working on social networking site where i've to load posts as and when user scrolls down. I've written following code to do that :-

$(window).scroll(function(){
    var post_count = "<?php echo $Posttotal; ?>";
    if($(window).scrollTop() + $(window).height() == $(document).height()){
        if(post_count >= 10){
            $("#lod_moreimg").show();
            loadmoreuserPost();
        }
    }
});

When i scrolling till the end of the page the load more function is not calling, but if i scroll to the top of the page then the load more function gets called. Can anyone know how to solve this problem? I want to load posts on scroll bottom but in my case the load posts working when i scroll up.