why HKDVD may be played in one DVD player and not in another DVD player?

[kakomu]

I can only suspect at this point. I haven't got up to date on DVD structure, so I really can't say. My guess would be multiple factors. First, Region Compatibility. some players may just reject Region 0 DVDs for the hell of it. Codec conformity: Some DVDs may have been encoded using a codec that is slightly out of spec or some sort of funky programming (may have been a menu, or some sort of file has been tampered with). These two problems can also be in conjunction with DVD player sensitivity. Cheaper DVD players are more sensitive, that is using hardware that recognizes only DVDs of Mpeg-2 Spec, and no deviations, so if there may be a slight problem with the DVD, the DVD player will have a fit and not recognize the DVD. Whereas better DVDs players (not necessarily more expensive however) will be able to recognize non-Spec codec encodes and play them just fine (better hardware with more expansive programming).

Like I said before, I'm no DVD officiando, nor do I have any real knowledge on the subject at hand, so this is all speculation, but that's basically all I can surmise.

P.S. - Glad someone read my guide :p