Why do some #start commands fail?

[llamabeast]

I've starting manually placing start sites recently, but I find that some proportion of them just don't work. If I put 10 start sites down for a 10 player game, maybe 2 or 3 won't work, which results in more than one player starting in some of the other start sites, leading some players to be instantly dead.

What I can't work out is what's wrong with the sites which won't accept the #start. Anyone have any ideas?