Thanks for the reply, Mike.
I've fixed the problem, although the detailed behaviour seemed to be slightly different for the two user records.
For one of those users, the user didn't map to the expected 'My Record' (this is on the central database). Updating a record on the corresponding 'client' database and a subsequent synchronization between that client user and the central user added a 'client user 2' user to the central database. That '<client user> 2' user did map to the '<client user>' contact 'My Record'. So I merged the users, retaining the '<client user> 2' user, and renaming it to '<client user>'. A subsequent synchronisation from the central database transferred that user to other client user databases.
There didn't seem to be that mismatch of users (on client and central databases) for the second problem user. In both instances, it seemed to require an amendment to a field in a Contact record that was recorded in the 'Notes/History' tab, referencing the user as the 'Record Manager'. I suppose this is just forcing the inclusion of the user in the next synchronisation packet that's sent to the central user database.
I suspect that the mis-matched user problem was because of the way the user was created on the central database. When creating the client user database from an empty copy of the central database, the user didn't exist. That client user was only created in the synchronisation set-up that followed.
That differs from the second user, which did exist on the central database when creating the associated client user database.
The successful distribution of users to other client databases just seems to have required a record that references such users.
