VCDs compatible in all players?

I have made a movie and converted to VCD using Nero Burner. But my client is asking for the VCDs to run in all players (VCD, DVD, Computers etc). He had problems with the previous VCD movie that it played in some VCDs and DVDs but not in some. So what might be the problem. Will my movie runs in all systems. If not, what would be the process to write VCDs so that I runs in all the system.

Some DVD players do not play VCDs simple as that. If he has one of these is does not matter what you do it will not play.

Even my old Sony claims to play VCD. I’ve never been able to confirm as it won’t play any CD-R. It will play DVD+/-R, but CD-R = no go.