It could be the quality of the DVD media. There is a lot of junk out there.

So how would that work, then?

Old PC + Ubuntu + Old DVD Drive = works

Dell PC + Ubuntu + New DVD Drive = doesn't work

Dell PC + Ubuntu + Old DVD Drive = doesn't work

The common factor is the Dell system. I can perform whatever DVD operations I want on (for example) my laptop.

 because junk is inconsistent

if you run DVD Identifier, you'll know the junk you have