jQuery Grid Plugin - jqGrid

I have a frustrating problem with the subgrid. When clicking the "+" to enlarge a subgrid, Firebug shows that data is being returned, but the subgrid expands empty. My subgrid model is based on a similar subgrid that works correctly. For debguing, I've reduced the subgrid to a single column (still no luck). Please advise.

Here is the code:

jQuery(document).ready(function(){
  jQuery("#list").jqGrid({
    url:'poquery.php?r_type=HIST_MAIN&TESTING=YES',   
    datatype: 'json',
    mtype: 'POST',
    height: 625,
    width:800,
    colNames:['POKEY','PO Number ',    'Date','Invoice','Buyer','Terms'],
    colModel :[
      {name:'POKEY', index:'HEFILE.DH_POHISTA.POKEY', width:45},
      {name:'PONUMB', index:'HEFILE.DH_POHISTA.PNUMB', width:45, formatter:fileLink},
      {name:'PODATE', index:'HEFILE.DH_POHISTA.PODATE', width:50},
      {name:'INVC', index:'HEFILE.DH_POHISTA.INVC', width:25},
      {name:'BUYER', index:'HEFILE.DH_POHITA.POINIT', width:70},     
      {name:'TERMS', index:'HEFILE.DH_POHITA.TERMS', width:70},     
     
    ],
    onCellSelect: function(rowid,iCol, cellcontent,target)
        {

        },
    pager: '#pager',
    rowNum:50,
    rowList:[10,50,100],
    sortname: 'PONUMB',
    sortorder: 'desc',
    viewrecords: true,
    caption: 'Purchase orders - Historical ',
    subGrid : true, subGridUrl: 'poquery.php?r_type=HIST_SUB&TESTING=YES',
    subGridModel: [{ name : ['Line'],
                    width : [50] } ], caption: "PO Detail"
  })
 
$("#s_button").click(function(){ jQuery("#list").searchGrid(
{sopt:['eq']} )});  
 
  $("#mysearch").filterGrid("list",{
        formtype:"vertical",

        formclass:"filterform",
filterModel: [
            {label:'LableFild', name: 'colname', stype: 'text', defval: 'default_value', surl: 'someurl', }],
           
        enableSearch: true,
        enableClear:false,
            searchButton:"Search",
        autosearch:false,
    });   

 });

Here is a sample of the data that is being pulled:

{"total":1,"page":1,"records":1,"rows":[{"id":"1","cell":["1"]}]}

No javaScript errors are being reported..

Please advise.