jQuery Grid Plugin - jqGrid

I am using jqGrid 3.44 and I am unable to get the formatter functionality working at all.  I originally used a custom formatter but the function call I put in didn't appear to ever be firing so I dropped back to just try a checkbox out of the standard formatter list.  My column continues to display as plain data.   I have double checked my include configuraiton to be sure fmatter is being included.  Below is my jqGrid code:

$("#list2").jqGrid({
    url: ajax_url,
    mtype: 'POST',
    datatype: "json",
    colNames:['Order ID#', 'Segment', 'Units','ACK', 'Tier', 'Bill To', 'Ship To', 'Order Date', 'Due Date', 'CSS Class', 'Over Due'],
    colModel:[ {name:'order_id',index:'order_id', width:90, jsonmap: 'order_id'},
          {name:'order_segment',index:'order_segment', width:70, jsonmap: 'order_segment'},
          {name: 'units',index:'units',width:50},
          {name:'acknowledgement', index:'acknowledgement',width:50},
          {name:'tier',index:'tier', width:50, jsonmap: 'tier'},
           {name:'billing_name',index:'billing_name', width:180},
           {name:'shipping_name',width:180},
           {name:'order_date',width:100,editable:true,formatter:'checkbox'},
           {name:'due_date',index:'due_date',width:100,editable: true},
           {name:'css_class',hidden: true},
           {name:'overdue',hidden: true}
         ],
   jsonReader : {
    root: "rows",
    total: "totalpages",
    records: "records",
    page: "page",
    repeatitems: false,
    id: "order_number"
    },
    
   pager: $('#pager2'),
   rowNum:100,
   rowList:[10,20,30],
   imgpath: gridimgpath,
   sortname: 'due_date',
   viewrecords: true,
   sortorder: "desc",
   height: 600,
   caption:"Open Orders",
   afterInsertRow: function(rowid, aData){
           if(aData.css_class != 'none')
           {
            for(cell = 0; cell < 9; cell++)
            {
             $("#list2").setCell(rowid, cell, '', aData.css_class);
            }
           }
           if(aData.overdue)
           {
            $("#list2").setCell(rowid, 'due_date', '', 'overdue');
           }
          },
   ondblClickRow: function( rowid ) {
           jumpToOrderID( rowid );
     } }).navGrid("#pager2", {edit: true, add: false, del: false});