Update problem with Forms 3.0

G'day,
Yet another problem with Forms V.3. I have created a form to modify a table called ASSETS, with a unique index REFERENCE. Now, I can happily add new records to this database, but I cannot modify the records. I have prevented REFERENCE from being modified, but the users are able to modify the other fields. But when I go to commit the changes, the database only updates the fields in memory...it comes up with the error : "FRM-40508: ORACLE error -- unable to INSERT record.". I could understand this if I was trying to add a record with the same name, but I'm only updating!!! Does any have a solution for this please (its driving me & the local support crazy). Oh, I've used the same SQL as in the form through SQL Plus and it worked. Weird.
Cheers,

--
  ,-_|\   *********** Rocqueforte Daemaen Alexander O'Leary *********** 
 /     \  B.App.Sci.(Comp),BPFH,Uni. Of Western Sydney,Nepean,Australia
 \_.--_/  E-Mail: roleary_at_st.nepean.uws.edu.au  Work Phone: 047 360 773 
      o           "Don't ask me how it works or I'll whimper"