jQuery Grid Plugin - jqGrid

Hello...
I have a nice little grid going, and I'd like to add in a 'grid as
subgrid' to view details of each record.
I have a function which looks something like this (see below) - now
the column where the plus symbol should show up is there, but with
nothing in it - and clicking on the empty column does nothing.
Inspecting the element with Firebug shows '<td style="" role="grid"/>'
So...any idea what I'm doing wrong here?

Using version 3.5.2, with a custom style.

function show_grid(fid,start,end){
    // Clear the grid of any information
    jQuery("#events").GridUnload();

    // Create a new jqgrid with new column names and modals.
    var mygrid = jQuery("#events").jqGrid({
        url:'genie.pl?
REPORT=flume_losses&PAGE=HTTPResponse_events.htmp&operation=get_grouped_events&fid='
+ fid + '&start=' + start + '&end=' + end,
        datatype: 'json',
        mtype: 'GET',
        height: 180,
        autowidth: false,
        colNames: column_names,
        colModel: column_modal,
        pager: jQuery('#pager'),
        rowNum:2,
        rowList:[2,30,50,100],
        sortname: 'event_count',
        gridview: true,
        viewsortcols: true,
        sortorder: "desc",
        viewrecords: true,
        imgpath: '/common/css/jquery-ui-mes-theme/images/',
        caption: 'Events detail',
        multiselect: false,
        subGrid:true,
        subGridRowExpanded: function(subgrid_id, row_id) {},
        subGridRowColapsed: function(subgrid_id, row_id) {}
    });

  return;