cloneDVD and DVD-R problem



I just got a DVD+/-R/W device and have started exploring the possibilities. I have bought both DVD-R and DVD+R (Memorex) media and successfully copied a few movies. The problem occurs when I’m using cloneDVD to write to DVD-R media, where I get a 'bad media " error. When I do the same with DVD+R it works fine. When I use DVDdecrypter for writing, the DVD-R work as well. Is there something special to consider using DVD-R with cloneDVD or did I grab some bad discs by chance?