dvd only plays some dvds

Thats not much to go on..

Did this just start happening?
Is it a new dvd drive?
what are you trying to play?
what model DVD rom is it?
Have you tried the same dvds that give you the problem in another dvd player? does it work?
Have you installed anything new lately?

In any case it could be a few things sounds like the DVDrom may be dying, software/firmware corrupted, Copywrite protection could be causing problems... Without more information its hard to tell.

A DVD Movie Player?

If it is a drive from a computer what type of Disc's are giving this error and probably more importantly what OS is this on?

Are these recorded or manufactured Disc's?

If this is in a computer it is possible that the Drive can not read these DVDs if they are recorded and on an Old Type Dye Lot. What can happen here is that the Dye Lots have changed color over the years so they do not reflect the light from this DVD Drive so while the individual DVDs may be all right they do not work with this Computer Drive.

If this is a DVD Movie Player for a TV or Home Theater system the format of the Disc may be incorrect for the player ideally any DVD Player used in these conditions should be DVD+/-R/RW/MP3/CD not just DVD-R which some of the older DVD Players where. If this player id DVD-R it will not read or play +R +RW and maybe -RW Disc's or any of the other formats that are used commonly these days.

Also if a Disc is left in the drawer constantly the LED that provided the Laser Beams will loose intensity and it will eventually stop reading any Disc's but before it gets tot hat stage it is possible that it will only read Manufactured Disc's and not any Recordable Disc's before it fails completely. The same drive should read all CD Formats as there are 2 different Color/Wavelength Lasers in these units so if it only reads one type of Disc and not the other type then that Laser Beam has failed and the player needs replacing.

Col