If the DVD is clean and has no scratches, then it's either 2 things:
1. You aren't using the last version of DVD Decrypter (version 3.5.4.0).
or
2. It's using a newer copy protection system (Howl's Moving Castle for example).
You can find DVD Decrypter 3.5.4.0 at
www.doom9.org or get it as part of
AMVapp version 3.0 beta. WARNING! Installing AMVapp 3.0b in the middle of a video project may bork any current projects you may have. That being said, you can select to only install DVD Decrypter from the AMVapp 3.0b installer by unchecking everything else in the installation setup.
Newer versions of the copy protection change things around so you may need to use something like
RipIt4Me to deal with it. Yes, it uses DVD Decrypter as a component and there is a download link for DVD Decryper 3.5.4.0 there on the page somewhere also.