Yep thats what I get for the most of the FujiFilm 8x DVD+Rs from Best Buy I got last week. Very few pass "judged as good" on 16x media check, most pass on 12x a few only on 8x.
Thats not a problem with the Plextor drive, no RMA needed. Thats just how 8x media quality varies. Plextor just allows you to try to burn at 16x, they don't guarantee it will work all the time with 8x media! The FujiFilm media is rated at 8x, so they don't guarantee above 8x burning either.
Use Verbatim 16x DVD+R media if you want to get higher quality 16x burns, or just burn the cheaper Fuji 8x media at 12x or 8x, its not that much slower but high quality at those speeds. By scanning with Plextools Media check first at least you can determine which speed is most likely needed for a good burn first.