dvd not recognized

[coffeefilter]

windows xp sp2 will not recognize certain dvds. cds play and most dvd
movies. there are a few dvds that read as empty when explored to the dvd
drive. these dvds are guitar instructional dvds from different publishers
and a few dvds i get from netflix.. this has just started. a year ago i
didn't have any problem. the dvds are okay as i can play them in my standard
dvd unit that is connected to the tv. even duplicating software will not
recognize and show as empty. i think i'm up to date with my video codecs.
i've tried playing these dvds on ms media player 11 and windvd 7...with no
luck. dvd drive has latest firmware. any ideas?

thanks,
dave