using birthdate does not catch duplicates

i used a unique fields combination trigger that should be checking for birthdate, family name, first name, middle name and recruitment agency. the trigger does not catch the duplicates, but when birthdate is removed from the combination, it catches the duplicates. also tried using validate record data that uses a query to check the 5 items above. this doesnt catch duplicates, but again, when birthdate is removed, it catches the duplicates. what's the best way to use birthdate (in combination with the 4 other items) to check for uniqueness?