When you say "existing customer" does that mean you migrated the customers into your new Kartris shop from somewhere else? If so and you used the Data Tool it might be that it hasn't set the ADR_TYPE to 'u'. If this is left blank then the migrated addresses don't show up but ARE actually in the database. I've just come across this myself when testing a migration from a Cactushop v6 database.
Simply running an SQL update like the one below should set the type correctly for all the "existing" ones (ie the ones without a type set).
UPDATE [yourkartrisdbname].[dbo].[tblKartrisAddresses]
SET ADR_TYPE = 'u'
WHERE ADR_TYPE = null