Identity column in oracle changes to none

Question

I've been dealing with this problem for some time now. I have a table with a column that acts as ID, I need a sequence of numbers to identify each one. So I'm using the Identity column feature in Oracle 11g, and it works fine, but after a little while, when I insert a new row I get the "ora-01400 cannot insert null into..." error, so I go and check if my identity column is still "configured" that way, and it's not. I don't know, am I maybe skipping a step?

The code for the sequence that gets created is:

CREATE SEQUENCE  "ELABOR"."WF_CONF_SEQ4"  MINVALUE 1 MAXVALUE 9999999999999999999999999999 INCREMENT BY 1 START WITH 981 CACHE 20 NOORDER  NOCYCLE ;

And this is the code for the trigger:

create or replace TRIGGER WF_CONF_TRG4 
    BEFORE INSERT ON WF_CONF 
    FOR EACH ROW 
    BEGIN
      <<COLUMN_SEQUENCES>>
      BEGIN
        IF INSERTING AND :NEW.ID IS NULL THEN
           SELECT WF_CONF_SEQ5.NEXTVAL INTO :NEW.ID FROM SYS.DUAL;
        END IF;
      END COLUMN_SEQUENCES;
END;

Does anyone know what could be wrong? I appreciate your help! Have a great day.