Not able to filter and sort columns using Json string from ajax source on ASPX page

Here's my java script:

$(document).ready(function () {

var table;

$("thead input").keyup(function () {
    /* Filter on the column (the index) of this element */
    table.fnFilter(this.value, table.oApi._fnVisibleToColumnIndex(
                    table.fnSettings(), $("thead input").index(this)));
});

$("thead input").each(function (i) {
    this.initVal = this.value;
});

$("thead input").focus(function () {
    if (this.className == "search_init") {
        this.className = "";
        this.value = "";
    }
});

$("thead input").blur(function (i) {
    if (this.value == "") {
        this.className = "search_init";
        this.value = this.initVal;
    }
});



table = $('#gridTable').dataTable({
    "bPaginate": true,
    "bProcessing": true,
    "bServerSide": true,
    "bSortCellsTop": true,
    "sDom": 'T<"clear">Rlfrtip',
    "oLanguage": {
        "sSearch": "Pesquisar no resultado geral:"
    },
    "contentType": "application/json; charset=utf-8",
    "sAjaxSource": "CustomerService.asmx/GetList",
    "fnServerData": function (sSource, aoData, fnCallback) {
        $.ajax({
            "dataType": 'json',
            "contentType": "application/json; charset=utf-8",
            "type": "GET",
            "url": sSource,
            "data": aoData,
            "success":
            function (msg) {

                var json = jQuery.parseJSON(msg.d);
                fnCallback(json);
                $("#gridTable").show();
            }
        });
    }

});

});