Hi Johanna,
The contacts you can see in Service Desk Manager and Service Catalog are stored in the ca_contact table in the mdb database. When you log into USS for the first time, the user will be created in the USS database( USS_mdb), not the mdb database... specifically in the User_ table ..
So if you look for the user in USS, you are searching in the USS_mdb/User_ table not in the ca_contact and this is why you cannot find the user... If you cannot find the user, that means that the user was never created (never logged into USS)... You could try searching at db level. Another good option would be to log into EEM with the user who¿s having the issue to confirm the password is correct.
Hope this helps.
Regards,
Pablo