I have strange problems playing my VCDs in DVD players.

I have a collection of VCDs which I made either by ripping from an original DVD or by directly copying from an orginal VCD. Some of my VCDs can be played in a DVD player, some cannot; even though they are in the same format.

For example, I bought a VCD movie that has 2 discs. I then copied these 2 discs onto 2 different discs and gave the original discs to a friend. One of the discs I made can be played in a DVD player, the other cannot. Both of these discs can be played in a CD-ROM or in a DVD-ROM.

What could be the reasons?