lock object does not work

I created a lock object 'ezvso', and function 'ENQUEUE_EZVSO' and 'DEQUEUE_EZVSO' were generated.

and i used the two function to lock and release the object in my program, it is always successful,

except one circumstance, as follow:

1. call function to lock the objectsuccessfully,

CALL FUNCTION 'ENQUEUE_EZVSO'

   EXPORTING

        mode_ztso_head       = 'E'

        mandt                = sy-mandt

        zvvbeln              = pv_zvvbeln

*       X_VBELN              = ' '

        _SCOPE               = '3'                           " i tried 1,2,3 ,all the same

*       _WAIT                = ' '

*       _COLLECT             = ' '

      EXCEPTIONS

        foreign_lock         = 1

        system_failure       = 2

        OTHERS               = 3    .

   IF sy-subrc <> 0.

     MESSAGE ID sy-msgid TYPE 'I' NUMBER sy-msgno

             WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4 DISPLAY LIKE 'W'.

     lv_7002 = '1'.

   ENDIF.

2. then, call function 'BAPI_SALESORDER_SIMULATE'

   the lock was released automatically,  how can i solve the problem?

BR!