jQuery Grid Plugin - jqGrid

function GetAttachment(callID) { //here i am getting right value
//$('#ActivityGrid').jqGrid('clearGridData');

// $("#ActivityGrid").jqGrid('setGridParam', { datatype:'json'}).trigger("reloadGrid");

var ActivityGrid;
//var myDataSuccess;
ActivityGrid = $("#ActivityGrid").jqGrid({
    ajaxGridOptions: { contentType: 'application/json; charset=utf-8' },


    loadError: function (xhr, st, err) {
        debugger;
        $("#ActivityGridMap").html("Type: " + st + "; Response: " + xhr.status + " " + xhr.statusText);
    },

    datatype: 'json',
    url: 'wts.asmx/GetAttachment?callID=' + callID, //passing correct value,but in webservice it gives me first value.
    mtype: 'POST',
    forceFit: true,
    loadComplete: function (data) {
        //  debugger;
        $(this).HideBusy();
    },
    serializeGridData: function (postData) {
        if (postData.searchField === undefined) postData.searchField = null;
        if (postData.searchString === undefined) postData.searchString = null;
        if (postData.searchOper === undefined) postData.searchOper = null;
        if (postData.filters === undefined) postData.filters = null;
        return JSON.stringify(postData);
    },
    jsonReader: { repeatitems: false, root: "d.rows", page: "d.page", total: "d.total", records: "d.records" },
    colNames: ['Activity', 'Date', 'Time', 'Handled By', 'Notes', 'Details', 'Attachment', 'Display'],
    colModel: [
                    { name: 'ClgCode', index: 'ClgCode', align: "left", width: 80 },
                    { name: 'Date', index: 'Date', align: "left", width: 70, formatter: 'date', formatoptions: { srcformat: 'yyyyMMdd', newformat: 'd-M-y' }, width: 100 },
                    { name: 'Time', index: 'Time', align: "left", width: 50, formatter: 'time', width: 100 },
                    { name: 'HandledBy', index: 'HandledBy', align: "left", width: 80 },
                    { name: 'Notes', index: 'Notes', align: "left", width: 120 },
                    { name: 'Details', index: 'Details', align: "left", width: 120 },
                    { name: 'Attachment', index: 'Attachment', jsonmap: "Attachment", align: "left", width: 0 },
                    {
                        name: 'Display', index: 'Display', align: "center", width: 100,
                        formatter: function () {
                            return " <input id=\"uxViewAtt\" class=\"MyButton\" type=\"button\" value=\"Display\"  onclick=\"return OpenAttachment(event);\"/>";
                        }
                    }
                ],
    rowNum: 5,
    rowList: [5],
    pager: "#ActivityGridMap",
    viewrecords: true,
    gridview: true,
    rownumbers: true,
    height: 60,
    width: 940,
    caption: 'Attachments'
}).jqGrid('navGrid', '#ActivityGridMap', { edit: false, add: false, del: false, search: false });