You have to be careful with usernames. They can exist in multiple forms
Common: Richard Schwartz
Abbreviated: Richard Schwartz/RHSC/US
Canonical: CN=Richard Schwartz/O=RHSC/C=US
The search string that you pass to @DbLookup has to match the format of what is stored in the view - and it can matter whether the view is referencing a name stored in a text field or a name stored in names field.
The @Name function provides options to [Abbreviate] or [Canonicalize].