RDS Rowset not different *enough* for update?

I am using a DCOM server program to read and update an Access database
using
the Jet 4.0 driver.

I have 2 columns in a single row rowset - LastUse (Date/Time) and
LastwasMatch (Boolean).  Initial values in the database are LastUse=0 and
LastwasMatch=false.

If LastUse is blank and I assign the field on the client side then
mRowset.Update will update the record on the server side.  If LastUse has
a
value and I change that value on the client side then mRowset.Update will
not return an Err.Number but the database is not updated and the lock is
not
removed.  (The lock is removed if the record is updated.)

Any ideas as to why the dataset would behave this way?

Many thanks,

J