Hi modernmagic,
it's normal because when you add a user with SEBLOD an entry is created in the table #__cck_core which is the main table of SEBLOD, and all search are based on this table.
So other user account don't have this entry and can't be listed.
One way if you don't have too many users in this case, it's simply to edit their account in backend and save it, so it will create the entry.
An other way is to
export all users in a csv file, then remove all users from joomla backend, and then import the CSV file. Be sure to have a backup of your database before that.
Regards.
Lionel