jquery ajax is not working in content place holder

// JScript File
 
$(document).ready(function () {
    $contentLoadTriggered = false;
    $("#mainDiv").scroll(function () {
        if ($("#mainDiv").scrollTop() >= ($("#wrapperDiv").height() - $("#mainDiv").height()) && $contentLoadTriggered == false) 
        {
            $contentLoadTriggered = true;
            $.ajax(
            {
                type: "POST",
                url: "LoadOnScroll.aspx/GetDataFromServer",
                data: "{}",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                async: true,
                cache: false,
                success: function (msg) {
                $("#wrapperDiv").append(msg.d);
                $contentLoadTriggered = false;
                },
                error: function (x, e) {
                alert("The call to the server side failed. " + x.responseText);
                }
            });
        }
    });
});