Follow rolling56 and etp to download dvd identifier, update the firmware and Plextools.
Media brand name, such as Memorex, Fuji, TDK etc, is meaningless for the burner. DVD burners "see" the media by looking its MID (Manufacturer/Media ID). DVD Identifier can show you the MID.
For example, Fujifilm DVD-R 8x can have MIDs: ProdiscF01 (Mfg: Prodisc) or Fujifilm03 or TYG02 (Mfg: Taiyo Yuden/TY). Only the last one is recommended by Plextor.
Memorex 8x DVD-R can be ProdiscF01 or CMC MAG AE1 (Mfg: CMC Magnetics). Both or not recommended.
TDK 8x DVD-R can be TTG02 or TTH01, both Mfg by TDK and are recommended.
You can still burn a not-recommended media but you will get mixed result, sometime burns well, sometime failed.
Another thing is, the latest firmware 1.04 fixed some DVDÂ±R writing strategies, you might want to try it.