Write conflict with SQL 7 after import

I have a database on SQL Server 7.0 and I am using an Access 2000 front end.
In Access 2000, I wrote an append query to import data from an Exchange 5.5 mailbox. The query runs OK and all the fields are imported, but when I try to edit the importedrecords in Access, I get a “Write conflict” error and the record cannot be edited.
Earlier records that were not imported in this way can be edited without error in the same table.
How do I free up the newly imported records for editing?