How new are these movies? DVDDecrypter hasn’t been updated in quite a while now and development has stopped on the program. If you are using it for decryption and ripping as well as burning, then you probably aren’t removing all the encryption/intentional structural problems that are intended to prevent copying.
Try DVDFab HD Decrypter. It is the free section of DVDFab. As you use it, you will be using up the trial period of the main program, but the free decryption/ripping section will continue to work even after the trial. http://www.dvdfab.com/free.htm This program is still being updated and can rip in file or ISO format.
You should also change your burning program. ImgBurn is the updated burning engine from DVDDecrypter. It is free to download and use. You should also read the guides on how to burn video files or ISO’s to dual layer disks: http://forum.imgburn.com/index.php?showforum=4
Finally we come to your choice of media. My advice is to stop using Memorex or Ridata dual layer dvds. The only dual layer disks good enough for us to recommend on a regular basis are Verbatim. Burn them at 4x.