aoColumns doesn't show a toggle image column

I'm using DataTables as a data grid manager on my app. Everything works fine but until I'm equipped a hidden row info plugin. I tried many ways to change things but the first column of toggle image never showed up. Here's my code:

**Javascript**

    var oTable;
    /* Formating function for row details */
    function fnFormatDetails(nTr){
    	var aData=oTable.fnGetData(nTr);
    	var sOut='<table cellpadding="7" cellspacing="0" border="0" style="border:solid 1px red;padding-left:50px;">';
    	sOut+='<tr><td>Renderingengine:</td><td>'+aData[2]+''+aData[3]+'</td></tr>';
    	sOut+='<tr><td>Linktosource:</td><td>Couldprovidealinkhere</td></tr>';
    	sOut+='<tr><td>Extrainfo:</td><td>Andanyfurtherdetailshere(imagesetc)</td></tr>';
    	sOut+='</table>';
    	return sOut;
    }
    
    $(document).ready(function() {
    	oTable=$('#labour_show').dataTable({
    		"fnDrawCallback": function(){//row highlighter
    			  $('table#labour_show td').bind('mouseenter', function () { $(this).parent().children().each(function(){$(this).addClass('datatablerowhighlight');}); });
    			  $('table#labour_show td').bind('mouseleave', function () { $(this).parent().children().each(function(){$(this).removeClass('datatablerowhighlight');}); });
    		},
    		"bProcessing": true,
    		"bServerSide": true,
    		"sAjaxSource": "inc/_labour_show_ajax.php",                
    		"aoColumns":[null,null,null,{"fnRender": function(oObj){return "<a href='main.php?do=labour&action=edit&sid=" + oObj.aData[7] + "'>Edit</a>";}	}],"aaSorting": [[1, 'asc']]
    	});
    	
    	$('#labour_show tbody td img').live('click',function(){
    		var nTr = this.parentNode.parentNode;
    		if(this.src.match('details_close')){
    			/* This row is already open - close it */
    			this.src = "images/ico_expand.png";
    			oTable.fnClose( nTr );
    		}else{
    			/* Open this row */
    			this.src = "images/ico_collapse.png";
    			oTable.fnOpen( nTr, fnFormatDetails(nTr), 'details' );
    		}
    	});
    });

**_labour_show_ajax.php**

This is an arrays from a server-side script

    $aColumns = array('labr_uid', 'labr_fname', 'emplyr_comp','labr_sid');

HTML Code:

    <table cellpadding="0" cellspacing="0" border="0" class="display" id="labour_show">
    	<thead>
    		<tr>
    			<th></th>
    			<th>Passport</th>
    			<th>Name</th>
    			<th width="30%">Employer</th>			
    			<th>Edit</th>
    		</tr>
    	</thead>
    	<tbody>
    		<tr>
    			<td colspan="5" class="dataTables_empty"><img src="images/loading.gif" /></td>
    		</tr>
    	</tbody>

No matter I've tried. The first column never shown up. Please suggest.