jQuery Grid Plugin - jqGrid

I need do multiple sorting, but the jqgrid not show me the icons of the sort direction for each column. What is wrong in this code?

jQuery(document).ready(function() {

$("#grid").jqGrid(

{

datatype: function() {

$.ajax(

{

url: "OrdenDeProduccion.aspx/GetOrdenDeProduccion", //PageMethod

data: "{'pPageSize':'" + $('#grid').getGridParam("rowNum") +

"','pCurrentPage':'" + $('#grid').getGridParam("page") +

"','pSortColumn':'" + $('#grid').getGridParam("sortname") +

"','pSortOrder':'" + $('#grid').getGridParam("sortorder") + "'}", //PageMethod Parametros de entrada

dataType: "json",

type: "post",

contentType: "application/json; charset=utf-8",

complete: function(jsondata, stat) {

if (stat == "success")

jQuery("#grid")[0].addJSONData(JSON.parse(jsondata.responseText).d);

else

alert("Error al traer los datos desde json");

}

});

},

jsonReader: //Set the jsonReader to the JQGridJSonResponse squema to bind the data.

{

root: "Items",

page: "CurrentPage",

total: "PageCount",

records: "RecordCount",

repeatitems: true,

cell: "Row",

id: "Id"

},

colModel: //Columns

[

{ index: 'Id', name: 'Id', width: 10, align: 'Left', label: 'Id', hidden: true },

{ index: 'Horaentrada', name: 'Horaentrada', align: 'Center', label: 'Hora', width: 200, sortable: false },

{ index: 'Producto', name: 'Producto', align: 'Center', label: 'Producto', width:200 },

{ index: 'Cantidad', name: 'Cantidad', align: 'Center', label: 'Cantidad', width: 200, sorttype: 'int' },

{ index: 'Precio', name: 'Precio', align: 'Center', label: 'Precio', width: 200, sorttype: 'float' },

],

caption: "Tabla Orden de Produccion",

loadtext: 'Cargando datos...',

emptyrecords: 'No hay resultados',

rowNum: 10, // PageSize.

rownumbers: true,

rownumWidth: 40,

gridview: true,

pager: "#pager", //Pager

viewrecords: true,

//sortname: "Cantidad",

sortorder: "asc", //Default SortOrder.

viewsortcols: true,

width: "760",

height: "260"

}).navGrid("#pager", "#grid", { edit: false, add: false, search: true, del: false });

});

Thanks,

Regards!