[AccessD] Updateable Query - Access vs. SQL Server

I have an Access file which contains a LEFT JOIN query between two tables.
If the query uses table links to an Access BE, the query is updateable -
which it should be.  But if the query uses ODBC table links to a SS Express
database on my PC, then the query is not updateable.  

To compound the mystery, the same Access query using ODBC table links to a
standard SQL Server database with the same tables IS updateable!

Does anyone know what's up with this or how it can be fixed?

Thanks!
Dan