"This recordset is not updateable"

I'm sorry if this question has been answered before, but searching the
archives is painfully slow so I thought I'd just ask.

I'm in the process of converting an Access application over to Postgres,
unfortunately I've got to keep Access on the front-end for the time being
and this has been causing me some headaches, ohh yeah it might help to
mention that I didn't write the application I'm porting, nor am I an
Access/VB person I'm just the guy stuck working on it, I come from a Unix DB
background in Informix, Oracle, Sybase. The current problem is that I have
a form with a combo box as the only updateable field and when I try to
change the value I get "This recordset is not updateable". The data that
makes up the form comes from 4 tables, but the field that I need to be
updateable is from a table that I'm also selecting the primary key from.
Any thoughts or suggestions would be greatly appreciated. If I should
re-write the form in another fashion please let me know and I'll even give
that a shot.

Rob