Update:
I pulled up the Kubuntu desktop and attempted to play the DVD using the "Dragon Player" software, and viola it worked. So now I know that it's not the changer itself.
Does anyone know where I should be looking for errors from LMCE related to a failure to run the DVD? I've been searching logs /var/log/ directory, but I can' t see anything related to my problem.
Thanks,
Ernesto