6 Disc set Doesn't Read the Same

I just backed up 6 disc set of HBO series with CloneDVD. All playback great on my computer and a 7"screen dvd player/viewer. But on my stand-alone DVD player only 4 discs play and 2 discs read as “bad”. I also tried a second DVD player and the 2 discs were unreadable. Any ideas as to problem? I re-burned 2 discs at slower speed, but still have problem. Then I re-burned with just feature film (no extras), still no success.
Thanks,
J