A number of reasons, from the DVD players don't like the type of disk (DVD-R or DVD+R as examples) or the disk was burned as PAL when it should be NTSC (or vice versa).
I just tried it on my daughters Panasonic DVD/VCR player and it works, but not on my other non Panasonic players. How can I tell if it was burned as PAL or NTSC? How can I change it? I'm getting a different brand of DVD's today and will try them tonite.
Check your DVD player manual and find out if it accepts the type of disk that you burned.
Some players will list both DVD-R and DVD+R as acceptable.
If you are satisfied that the disk is the correct type, try some 'Verbatim' brand and burn the disk at a moderate (4x) speed.
As a side note, some players are more tolerant than others and will play disks that have a small amount of error burned in.
If you use Nero, you can scan the disk using the Nero tool "Nero CD-DVD Speed" and it will show if your disk is perfect or has some errors.
