RowGroup Not Working

I have a rowGroup which is defined as such;

            rowGroup: {
                //className: 'vendor_details',
                emptyDataGroup: null,
                dataSrc: 'ps2.segment_title',
                // startRender: function ( rows, group ) {
                //
                //     return group +' ('+rows.count()+' rows)';
                // }
            },

I have a column definition in my datatable (same one that im attempting to do rowGroup on) which is as such;

{"data":"ps2.segment_title"}

The segment_title is displaying correctly in the table however, the rowGroup does not work for this value. If I change it to another value in my dataset, it works fine. The value that it does work for is derived from the main table whereas ps2 is derived from a leftJoin. Not sure if this is the reason why it suddenly doesnt work. Any help would be much appreciated.