Special letter behaviour problem?
The question is a bit vague, but typically when you have Unicode characters and you end up with question marks or boxes, the problem is actually relatively simple. When you declare a Unicode string with 'single quotes', you need to prefix them with an N''. N stands for national, not nvarchar (admittedly, a U prefix - for Unicode -  would have made more sense).

Compare:

SELECT Prefix = N'', NoPrefix = '';


Results:

Prefix      NoPrefix
------      --------
           ??