Bibtex and Xelatex problems

Hello,
I have started to create a bibliography with BibDesk and some nice 
scripts that help me gather all the information. Now I wanted to 
actually print out the whole stuff and had to find out that some unicode 
characters are plain left out. That is especially for characters that 
use bars and dots under and above letters such as you would want for 
trasliteration of Arabic script.
I hope this is not an unsolvable problem, EndNote, Books and the like 
are really not that comfortable after all.

Thanks in advance,
Bernd.

PS: I use Bibdesk with Unicode enabled, TexShop and Xetex, always the 
latest version except Xelatex (.96?)