Are you perhaps running Access 2003 with the SP3 service pack installed? The
SP3 service pack and it's buggier than the Great Dismal Swamp! One of the
most common bug reported involves comboboxes with disappearing data! The
field(s) in the cbo, in the underlying table, have some kind of formatting in
place. This is often the simple > used to force UpperCase. Remove any
formatting from the fields and things should work short term, but the
definitive solution is to install the hotfix for SP3.
Here's a link to Allen Browne's site where you can find the latest news about
these bugs, as well a link to the Hotfix:
http://allenbrowne.com/bug-Access2003SP3.html