Trigger for autoinc ID

Hi all,
Excuse my English...
I'm working with Database Workbench (= DBW) on my Firebird for 9 month with great pleasure.
But now when I'm going to put the autoinc IDs on my tables I get problems.
I can put a trigger on my table and view it nicely in DBW as trigger on the table, but next time I refresh/connect the trigger has gone and the depending object on the generator is marked as a check symbol and if I try to create the trigger again I'm told it's existing.
I've already made this kind of trigger successfully on 12 tables with no problems.

create table CHECKELPOLSTAFF (
ID Integer NOT NULL,
StaffID Integer,
Dato Date,
Tid Time,
WhereWas varchar(20),
BreakTime Time,
CONSTRAINT PK_CHECKELPOLSTAFF PRIMARY KEY (ID)
);

CREATE
INDEX inxStaffID_CHECKELPOLSTAFF ON CHECKELPOLSTAFF (StaffID, Dato, Tid, WhereWas);

CREATE GENERATOR CHECKELPOLSTAFF_GEN;
SET TERM ^^ ;
CREATE TRIGGER CHECKELPOLSTAFF_ID FOR CHECKELPOLSTAFF ACTIVE BEFORE INSERT POSITION 0 AS
begin
if ( (new.ID is null) or (new.ID = 0) )
then new.ID = gen_id(CHECKELPOLSTAFF_GEN, 1);
end
^^
SET TERM ; ^^

Thanks
Pia