Use MediaInfo on the vobs from some DVDs that work and some that don't and see if you can spot any common differences in video/audio codecs, NTSC/PAL format etc.
If the movies that you are starting with play on your computer then it should be possible to make DVDs out of them. You could try some different software to see if that helps: AVStoDVD, DVD Flick, FAVC etc.
One other possibilty is to run the output from Nero through DVD Shrink before burning; it should only take a few minutes and will ensure that the VIDEO_TS folder meets the DVD-Video standard.
If your standalone supports DVD+/-RW then I recommend that you get a couple of disks for testing purposes to avoid making too many coasters.