PL/SQL FOR UPDATE lock inconsistent and invisible for other sessions

Question

I've written a simple cursor which locks a specific row for update. Under a large amount of attempts for multiple threads it appears that the lock does not work and other threads are able to update the locked row. As if they are not required to wait until the lock is released.

CURSOR cu_LockEntity (lockType    IN YSTEM_LOCKS_T.LOCK_CODE%TYPE,
                      entityType  IN SYSTEM_LOCKS_T.ENTITY_CODE%TYPE,
                      entityId    IN SYSTEM_LOCKS_T.ENTITY_ID%TYPE) IS
     SELECT LOCK_TYPE_CODE
       FROM SYSTEM_LOCKS_T
      WHERE LOCK_CODE = lockType
        AND ENTITY_CODE = entityType
        AND ENTITY_ID = entityId
        FOR UPDATE;

I use similar locks throughout the database and do not encounter such issues. What can be the reasons for this? Is there a more efficient locking option I can use?

This is the query that updates the lock for a new thread that wants it.

UPDATE SYSTEM_LOCKS_T
   SET LOCK_DATE = SYSDATE
 WHERE LOCK_CODE = lockType
   AND ENTITY_CODE = entityType
   AND ENTITY_ID = entityId;

Sorry that I cannot disclose the entire purpose of this lock.