Why can't I rip recordable DVD Disc?

I have a iMac and have downloaded the "MacTheRipper" feature which rips DVD's to where you can put them on your computer and/or itunes and for normal dvd's like movies and TV shows it works great but when I try to rip a recordable dvd it doesn't work...... does anyone know of a recordable dvd disc that would work on MacTheRipper? Or if you're even able to at all?