I just bougth a dvd burner. I burned a number of cd-rs. To my
surprise, my dvd player will play some of them, but not others. What
are the possible causes?

According to my personal experiences, one possible reason are bad
DVD-Rs. Inconsistent quality and flaws, even among DVD-Rs from one and
the same box, can be a huge problem.

Scriptor

You may want to check out your DVD players compatibilties at
http://www.vcdhelp.com.  Look up DVD player and it will tell you what
type of media it will play along with most likley some user reviews of
postitive and negative features.

Try buying some of the higher priced DVD-Rs and see how they work.

we sell dvds in www.inter-shop.gr/commerce/catalog.

we had problems with b class discs.
try wellknown brands as sony tdk arita traxdata fuji companies that do
the best not to spill their name with bad disc production.