DVD Roms won't read some DVDs

I am having a problem with my DVD Roms, 1 whatever-came-with-my-ACER and 1 Liteon external. Neither one seems to be able to read some of my DVDs.

They are all region 3, and besides some of my other DVDs read just fine.

Realplayer does nothing when asked to play and PowerDVD insists there is no disc in the drive. I have also tried Interactual and Windows Media Player.

My machine is running Windows only.

I have tried cleaning the dics but to no avail. Please help, I want to watch Lord of the Rings again.