why this jquery won't trigger .ajax() request?

i have this code

// jquery
$(document).ready(function() { 
    $('#update_point').live("change", function() {
        var point_val = $('#update_point').val();
        $.ajax({
            url: 'send/user/update_point.php',
            type: 'get',
            data: 'point='+point_val,
            dataType: 'json',
            success: function(data){
                alert(data);
                $('#update_point_result').html(data);
            }
        });
        return false;
    });
});

why that code doesn't fired ? but if i'm remove dataT开发者_开发百科ype, the code works. why ?

// jquery
$(document).ready(function() { 
    $('#update_point').live("change", function() {
        var point_val = $('#update_point').val();
        $.ajax({
            url: 'send/user/update_point.php',
            type: 'get',
            data: 'point='+point_val,
            success: function(data){
                alert(data);
                $('#update_point_result').html(data);
            }
        });
        return false;
    });
});

any help will be appreaciate it ! Thanks !

edit

update_point.php contains this code.

<?php
require "../../inc/json.php";
if($_GET){
    foreach($_GET as $key=>$val){
        $respon[$key] = $val;
    }
}

// initialitation json object

$json = new Json();
echo $json->encode($respon);
die();
?>

$.ajax likes to silently fail when JSON is badly formed. Check that the JSON from the server is well formed, using a tool such as jsonlint.

You can use the .error callback to check the type of error being thrown:

$.ajax({
        url: 'send/user/update_point.php',
        type: 'get',
        data: 'point='+point_val,
        success: function(data){
            alert(data);
            $('#update_point_result').html(data);
        },
        error: function(jqXHR, textStatus, errorThrown) {
            alert(errorThrown); // likely 'parseError'
        }
 });