bad dvd from iMovie?

[Donald Hinkle]

I had a brief iMovie (v. 4) a friend had made; needed an iDVD. In 
iMovie, after dropping all the Clips to the TimeLine, I clicked on DVD, 
got the Theatre with music, clicked Burn. After 20 minutes or so it 
said it was done. I handed it to friend who put it in DVD player; 
doesn't work. I tried it on my ALPB, where I burned it; we see the 
"preview," the first couple of shots in the theatre curtains, but Play 
doesn't work; there's apparently nothing more.

I'd like to redo it, but I'm concerned--what went wrong and how can I 
avoid it?
X 10.3.3 on ALPB.