When can an after insert/update trigger not fire

I've got this trigger

create or replace

TRIGGER "DOKCENTRA"."TR_INTDOC_AFTER" AFTER

  UPDATE or INSERT

    ON "DOKCENTRA"."AG_INT_DOC" FOR EACH ROW

  BEGIN

    IF UPDATING THEN

      IF (:NEW.DOCNUMBER IS NOT NULL AND :OLD.DOCNUMBER IS NULL) THEN

        INSERT

        INTO

          AG_DOCNUMBERS VALUES

          (

            :NEW.DOCNUMBER

          );

      END IF;

    ELSIF INSERTING THEN

      IF (:NEW.DOCNUMBER IS NOT NULL) THEN

        INSERT

        INTO

          AG_DOCNUMBERS VALUES

          (

            :NEW.DOCNUMBER

          );

      END IF;

    END IF;

  END;

Let me describe the situation first:

I've got two tables: AG_INT_DOC and AG_DOCNUMBERS . AG_INT_DOC has a column named DOCNUMBER.  This column gets its value in BEFORE INSERT/UPDATE trigger. I need to make sure that any value that is inside