What I would do, I would try mounting it manually as root if necessary:
mount /dev/dvd /mnt/dvd
If that doesn't work with the problem disk, but you can view other dvds without a problem, I would say the problem lies in the disk itself. I've found that if a disk is brand new without any smudges or scratches, my computer sometimes has trouble reading it. If I smudge the disk slightly they sometimes work. It's a crazy trick, but I don't know if it will work for you.