Burner Won't Burn on DVD-R

[GreenKronic]

I am having some problems when burning a DVD I don't know why but when I burn a DVD+R(RIDATA) it works fine no problems , But when I use a DVD-R(Memorex) it used to work when I burned using them now it doesn't at all. It starts to Burn then it stops and wasted my DVDs. I am using Nero 6 and tryed DVDsanta. I know my dvd burner can burn on dvd-r's. The Burner is Mad Dog MegaSTOR 7-in-1 16X Double Layer External DVD±R/±RW