try using dvd shrink or another alternate dvd ripper, or perhaps the dvd your attempting to pirate is protected by some sort of DRM that is preventing you from grabbing the data.
I know when I attempted to rip some db-gt once it wasnt possible, and i had to use a external player to physically play the dvd and then encode it from my tv capture card. but this is possibly not an option for you.
try with some different software.