If you use loadonce:true jqGrid change the datatype parameters to 'local' after the first load of data from the grid. All next grid reloading (sorting, paging, filtering) works local. If you want refresh the grid data from the server one more time you should set datatype to its original value ('json' or 'xml'). For example:

$("#list").setGridParam({datatype:'json', page:1}).trigger('reloadGrid');

UPDATED: Free jqGrid supports fromServer: true option of reloadGrid starting with the first release (starting with version 4.8). So one can use the code like

$("#list").trigger("reloadGrid", { fromServer: true, page: 1 });

to do the same as above. The main advantage: such code works fine with any initial value of datatype ("json", "jsonp", "xml" and so on). Free jqGrid saves original value of datatype inside of internal dataTypeOrg before changing it to "local".

One more helpful option of free jqGrid is the parameter reloadGridOptions of navGrid, which allows to specify default options of reloadGrid. Thus one can use for example

loadonce: true,
navOptions: { reloadGridOptions: { fromServer: true } }

options of jqGrid, which set defaults for navGrid additionally. As the result the click on "Reload" button of navigator bar will reload the grid from the server instead of local reloading.

Nice was trying for last one week , solution is perfect use

jQuery("#datalist").jqGrid().setGridParam(
    {
        datatype:'xml', 
        page:1, 
        url : '<%=request.getContextPath()%>/PreviewReport?cmd=1&fromdate='+vfromDate+'&todate='+vtoDate+'&status='+vstatus+'&keyword='+vkeyword+'&mdn='+vmdn+'&filetype='+vfiletype
    }
).trigger("reloadGrid");

to reload the data using loadonce:false

Just to say, for me, the following line wasn't enough, to refresh the data in my loadonce:true jqGrid:

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

After calling that line, I tried to call my code which loaded my JSON data and populated the jqGrid with it, but it didn't refresh the rows in my grid.

My solution was to forcibly unload the jqGrid, and then call my function to recreate it.

$("#MikesGrid").jqGrid('GridUnload');

Perhaps I was just unlucky.

Btw, when I get a chance, I'll document how I wrote a generic JavaScript function to add two buttons to any jqGrid, one to refresh the (loadonce) data, and a second button to export the jqGrid data into a real Excel file, using my library:

Export jqGrid to an Excel file

I like reuseable code !