jQuery有时$ _POST是空的

Sometimes i get report that in jQuery Ajax query $_POST is empty. For example Google Chrome on OS X or Windows 7 Firefox sometimes the post is empty sometime it isn't. I can't figure out what seems to be the problem

jQuery.ajax({
    url: "ajax.php",
    method: "POST",
    data: {
        jqueryAjaxPostData: JSON.stringify({
            name: "gen",
            id: 1
        })
    },
    processData: true,
    dataType: "json",
    failure: function ()
    {
        alert("Ajax failure")
    }
});

EDITED : There is PHP code

$post = Variable::toArray(json_decode($this->Http->post("jqueryAjaxPostData")));