SQL & PL/SQL

Hi,
I have this query which is returning the correct results

SELECT hdr.order_num
               ,hdr.business_unit_id
               ,dtl.item_class_id
           FROM xxx.dd_ddhdr hdr
               ,xxx.dd_dddtl dtl
          WHERE hdr.order_num = dtl.order_num
            AND hdr.business_unit_id = dtl.business_unit_id
            AND dtl.item_class_id NOT IN 
            ( 
               SELECT cfl1.filtered_column_value 
                 FROM sdds.ai_client_filter_list cfl1
                WHERE cfl1.client_id = 'XXT' 
                  AND cfl1.filtered_column_name = 'ITEM_SUBCLASS' 
                  AND cfl1.business_unit_id = hdr.business_unit_id
            )
            AND hdr.order_datetime 
            BETWEEN to_date('3/28/2007 13:21','mm/dd/yyyy hh24:mi:ss') 
                AND to_date('3/28/2007 13:26','mm/dd/yyyy hh24:mi:ss');

I converted the above query to use NOT EXISTS instead of NOT IN as follows:

SELECT hdr.order_num
               ,hdr.business_unit_id
               ,dtl.item_class_id
           FROM xxx.dd_ddhdr hdr
               ,xxx.dd_dddtl dtl
          WHERE hdr.order_num = dtl.order_num
            AND hdr.business_unit_id = dtl.business_unit_id
            -- AND dtl.item_class_id NOT IN
            AND NOT EXISTS 
            ( 
               select 1 
                 from sdds.ai_client_filter_list cfl1
                where cfl1.client_id = 'XXT' 
                  and cfl1.filtered_column_name = 'ITEM_SUBCLASS' 
                  and hdr.business_unit_id = cfl1.business_unit_id 
            )
            AND hdr.order_datetime 
            BETWEEN to_date('3/28/2007 13:21','mm/dd/yyyy hh24:mi:ss') 
                AND to_date('3/28/2007 13:26','mm/dd/yyyy hh24:mi:ss');

The 2nd query does not return any row after converting to NOT EXISTS.

Any idea to solve this?.
Thanks!