my ajax query doesn't work time to time!!!

my ajax query doesn't work time to time!!!

Hello everyone ;
I'm writing an application with PHP that let me have statitics about visited pages for my web site. to save informations needed i use an ajax query with the unload event. The problem that i have is titme to time the script uses with that ajax query doesn't work especially when i stay long time in a page.
this is my code?

<script type="text/javascript"> 
 
 function closeIt() { 
$.ajax({ 
async: "false",
url : "out.php", 
success : function(data) { 
} 
}); 
 } 
  
$(function(){
 $(window).bind('beforeunload ',function(){
 //$.post( "out.php"); 
 closeIt();
 }); 
})
</script>

why it works most of time but sometime doesn t work???
is there any specifications to take for the unload event ??
Thanks for your help.