own dvd copy won't backup

I transformed a couple of my home made video films into DVDs
successfully with my my g4, OS X.2.1, imovie and idvd 3.0.1.
As the result is quite nice - the dvds perform smoothly on commercial
players- , lots of my friends wanted to have a copy of the work. I had
already skipped the clips and other items from my hard drives, but I
thought, it would be the easiest thing, to make copies with the disk
copy application, that came with my system software. In fact it was,
but the dvds won't play on any commercial dvd player nor on my G4
under OS X. On the other hand they DO WORK, on my G4 under System
9.2.2. , which means, to my humble oppinion, that the other players
respectively my G4 under OS X won't playback those disks, BECAUSE they
have been COPIED. This would be fine with me, if it was pirated films,
but it is my own intellectual property, which I seem not to be allowed
to copy. What did I do wrong, when I produced the master disk or what
do I do wrong, when I try to multiply MY DVDs.
Does anyone of you folks know, where the catch is.....?

Wolf