Can be a number of things so can only generalize:
1. DVD players support certain media/brands i.e. DVD+r/-R
2. must be standard compliant
3. must be accordance to manufacturer specs or dvd player
4. Wasn't burned properly
I suggest you look at manufacturer/model of DVD player and find out what it supports:
http://www.videohelp.com/dvdplayers.phpAlso, consider reading some guides/info resources I posted in this thread:
http://www.click-now.net/forums/index.php?showtopic=138