Can't Add Record in Subform

Hi, folks. This USED to work, but I can no longer add a new record to a
subform, and I'm not sure what I changed to screw it up.

I have an SQL Server 2000 database which is a modified version of the
Northwinds DB. I have it linked using Linked Tables to an Access 2003
database. All the tables are in the SQL backend; Access handles all the forms
and reports.

I have a CUSTOMERS table, similar to the one in Northwinds. I have added a
MAILING LIST CONTACTS table in SQL, linked by CustomerID. (Clear as mud,
right?) The CUSTOMERS table contains all the info about each client company;
the MAILING LIST CONTACTS contains name, mailstop, etc, for employees at that
client company.

The CUSTOMERS form is based on the CUSTOMERS table, of course. I created a
subform to list the employees in the MAILING LIST CONTACTS table, and I used
this to add the names & etc for each contact.

But I've changed something, and now I can't add or edit any of the subform
contacts. I can add new CUSTOMER records, but can't add or edit any mailing
list data. If I use SQL Enterprise Manager, I can add/edit data that is then
visible in Access, but can't edit it in Access.

Any suggestions? Thanks!