We have a problem with a JDBC2 driver that's driving me nuts!

We have a connection between our IDVAULT and a MS SQL database. The driver
schema is very simple:

GUID = PK_Account_ID
Surname = lastName
Given Name = firstName
CN = username

When we migrate the users (of which there are thousands) all work perfectly
well apart from 7 or 8, who are provisioned into the SQL table with all
fields complete apart from 'username' (which should be set to the CN of the
user, as it is with the remaining users.

I have narrowed down details of the 7/8 users that do not have username set.
The one thing that is identical to them is that the CN value when
manufactured is identical to the surname of the user, ie Steve Edwards would
become EDWARDS. The user would have a row created in the SQL table, and all
values would be set apart from the username field.

Has anyone any ideas why this might be happening? It's only a few users out
of thousands but it's running me round in circles trying to find out where
the problem is.

Thanks for your help!