Oracle, poor performance

below query in our process which is taking between 7-8 seconds when executed. when you execute the same query by commenting out the condition AND INBOUND_OUTBOUND_IND = 'I', it returns within milli seconds. The column INBOUND_OUTBOUND_IND is partitioned. we included the column in the condition to gain performance (searching 1 million records vs total 3 million records). other queries using INBOUND_OUTBOUND are just working fine, but this one is not. Could you please review and let us know why the condition on this column which is supposed to gain a performance is hurting us. If it can be fixed by defining any indexes on the database side, please let me know. any help in this regard is greatly appreciated.

SELECT inbound_seq_pk
      FROM aqtit01.tqt_it_inbound_seq
     WHERE inbound_seq_pk = (SELECT inbound_seq_pk
                           FROM (SELECT inbound_seq_pk, rownum AS rn
                                   FROM Aqtit01.TQT_IT_INBOUND_SEQ inboundSeq
                                  WHERE Inbound_Seq_Pk IN (SELECT Inbound_Seq_Pk
                                                             FROM Aqtit01.TQT_IT_Inbound_Parent
                                                            WHERE PARENT_MATRL IN (SELECT PARENT_MATRL
                                                                                     FROM Aqtit01.TQT_IT_Inbound_Parent
                                                                                    WHERE Inbound_Seq_Pk = 3349801) )
                                    AND Seq_Nbr     < 20110927191030002
                                    AND STS_CDE not in ('RESPONSE_SUCCESS')
                                    AND INBOUND_OUTBOUND_IND = 'I'
                                  ORDER BY seq_nbr
                                ) a
                          WHERE a.rn = 1)
                          AND STS_CDE not in ('RESPONSE_SUCCESS')