Locate on Max() and Group By

I have the next problem:

I open an table with

select dnkey, Max(dvnaam) as dvnaam2, Max(dvpersoonnr) as dvpersoonnr   from
tbmedewerker
group by dnkey

When I want to locate like

    TblTbPGSMedew.Locate( 'dvnaam2', 'Hallo', [ ]);

then 'Hallo' isn't find

but when I open the table without Max() en group By like

select dnkey, dvnaam as dvnaam2   from tbmedewerker

then 'Hallo' is find.

How can I fixed this problem?
I working with PostgreSQL 7.1 and Delphi 5