Sorry if this has already been posted. I didn't see it anywhere when I looked.
Basically the problem occurs after one (or more) Ctrl-N to change the galaxy setup at the beginning of the game. The other races don't produce anymore than the colony ship they start with, and don't take over any more un-colonized planets near their home world.
This isn't a major deal, but is a little annoying, especially when you find a nice galaxy-planet setup to play...and the other races don't play back

I have a saved-game that can be loaded if somebody wants to address the issue.