I have a Matshita UJ-840S and it's extremely picky about DVD media. Most DVD media it doesn't like at all, producing burns that are hard to read in some drives or even discs that aren't even recognized.
Of the media I have tried so far, it only really likes YUDEN000 T03 (Taiyo Yuden 16x DVD+R) and MCC 004 (Verbatim 16x DVD+R).
Have you tried using some other DVD media?
Pioneer DVR-112L, LiteOn SHM-165P6S, Pioneer DVR-111L, BenQ DW1655, Plextor PX-760A
LiteOn LH-20A1P, Optiarc AD-7203A, Matshita UJ-840S, Sony RDR-HX910, Sony RDR-HX750