Sometimes concurrent request is running very slow or could not able to complete so user have to handle those request through follow few steps.

Find out the request which is not working perfectly

SELECT *
  FROM v$session
 WHERE process IN (SELECT os_process_id
                     FROM fnd_concurrent_requests
                    WHERE request_id = '&request_id');

Get SID from above sql and find object_id's from below script

SELECT *
  FROM v$locked_objects
 WHERE session_id = '&SID'

Now take all the object_id's and check any other session is locking those object_ids apart from the above SID.

SELECT *
  FROM v$locked_objects
 WHERE object_id = '&object_id'

If any session is locking check the status of that session from v$session.
If status is inactive kill that session to free up the resource

ALTER SYSTEM KILL SESSION 'SID,SERIAL#';