My vote is poor quality disks. The only dual layer that is regularly recommended around here is Verbatim +R DL made in Singapore.
From user reports, the Verbatim DL made in India is less consistent.
Which burner are you using? If it is a fairly new one, you can use either the older mid code MKM001 or the newer mid code MKM003, which are 8x DL Verbatim. If your burner is a couple of years old, or older, I would stick with the slower speed MKM001 Verbatim DL, which are 2.4-4x speed I believe.
4x is usually a safe speed for burning either variety of Verbatim DL, though some people prefer 6x for the newer higher speed version.