Can´t copy dvd 2 dvd



I use nero 6.3.16 and i can’t copy dvd 2 dvd. I’ve got 2 plexies the reader is the PX116A2 and the writer is the PX708-A, and whwn i try to make a direct copy it gives me an error saying i cannot copy that dvd. The dvd is not copyrighted so i dunno if it’s a nero thing, an windoze problem, or plextor problems. The strange thing is that whem i use Nero 5.5 there is no problem.
Any suggestion would be much aprecciated.
cheers and thanks for input