PGMusic tech support got back in touch with me and said that some of their content is sent out on double-layer DVDs and that, apparently, some drives just don't work well with them. So, they're sending out replacements for me on single-layer DVDs.

Checking out the DVDs I have, all the ones that loaded fine had matte white labels and the all ones that didn't load had shiny silver labels. The latter must be the double-layer ones. (Hope that info will help someone else out...)

Again, problem seems to be solved, as soon as I get the new disks. I'm anxiously awaiting....