do you watch it in the same player?
if so, my guess would have to be that if it doesn't play, its because your
DVD player is older than the DVD format its written in.
For example, my parents DVD player is one of the originals, and it only plays bought DVDs. But if i had copied a movie on to a DVD+R disc, and tried to play it in their player, it will not play. This is because the DVD+R format is a relatively newer format [newer in comparison to DVD-R].