No need to post in bold, our collective eyes are good enough to read regular font...
The only thing I can think of is, the various DVD player manufacturers, if they don't make their own drives (and even if they do, my comments will still apply) get the drives from other drive makers, and build them into their machines.
This can (and does) cause a wide variety of drive sensitivity and affects their ability to read the reflective layers on the DVD blanks.
Without picking on any one DVD player Mfr., the fact that some of your players can read the Memorexes means, basically, that their lasers are set (or built) in such a way as to read faint write tracks. Others are not set in such a way, or are manufacturered at a level that won't allow them to read the brands of blanks that have a lower reflectivity co-efficient.
Which is a wordy way of saying, stick with the quality brands of media - you'll have far fewer coasters and problems.