เพราะสาเหตอะไรครับ บางครั้ง Ajax login เวลาเอาขึ้น server แล้วมันไม่ทำงานครับ

<?php
session_start();
$username = $_POST['name'];
$password = $_POST['pwd'];
//$password = md5($_POST['pwd']);
$mysqli=mysqli_connect('localhost','root','ddsure','fts');

$query = "SELECT * FROM tbl_staffs WHERE member_username='$username' AND member_pass='$password'";
$result = mysqli_query($mysqli,$query)or die(mysqli_error());
$num_row = mysqli_num_rows($result);
		$row=mysqli_fetch_array($result);
		if( $num_row >=1 ) {
	
		
			echo 'true';
			$_SESSION['user_name']=$username;
			$_SESSION['istaff'] = $row['member_code'];
		$_SESSION['iname'] = $row['member_name'];
		$_SESSION['ilname'] = $row['member_lastname'];
		$_SESSION['istatus'] = $row['member_status'];
		}
		else{
			echo 'false';
		}
?>

<script type="text/javascript">
$(document).ready(function(){
	$("#login_a").click(function(){
        $("#shadow").fadeIn("normal");
         $("#login_form").fadeIn("normal");
         $("#user_name").focus();
    });
	$("#cancel_hide").click(function(){
        $("#login_form").fadeOut("normal");
        $("#shadow").fadeOut();
   });
   $("#login").click(function(){
    
        user_name=$("#user_name").val();
        pass_word=$("#password").val();
         $.ajax({
            type: "POST",
            url: "login.php",
            data: "name="+user_name+"&pwd="+pass_word,
            success: function(html){
              if(html=='true')
              {
			   location.reload();
			   $("#name").val();
                $("#login_form").fadeOut("normal");
				$("#shadow").fadeOut();
				$("#profile").html(" <div id='profile'><div id='shadow' class='popupok'></div></div>");
				
              }
              else
              {
                    $("#add_err").html("Wrong username or password");
              }
            },
            beforeSend:function()
			{
                 $("#add_err").html("Loading...")
            }
        });
         return false;
    });
});
</script>

user_name=$("#user_name").val();
pass_word=$("#password").val();
$.ajax({
type: "POST",
url: "login.php",
data: "name="+user_name+"&pwd="+pass_word,

var user_name=$("#user_name").val();
var pass_word=$("#password").val();
$.ajax({
type: "POST",
url: "login.php",
data: {name:user_name, pwd:password},