DVDs don't work

I'm not sure what the problem is - but the DVDs that I burn playback fine in some units, but not in others.
The rate of discs that do not work is rather high.

Details:
DVD Studio 4
G5 2.0ghz, 2Gig of Ram
LaCie External writer, as well as the G5 internal drive

When my project is finished, I hit the yellow/black burn button and pop the disc in.
It plays fine in my computers (Mac and PC), cheapie DVD home units and the more expensive ones.

But i consistently hear from friends and family, either or; they do play or they don't.
I get the same results whether I use the internal drive, or the LaCie.

I appreciate any and all feedback.

Best,
Jim
West Palm Beach, FL