Are the other races having the same problem as you or do they have more than two planets? If they have more planets, you are probably too slow to colonise. I doubt that, so let's move on:
Set habitable planets to 'abundant', number of planets planets to 'abundant' and make sure you don't stuff a tiny map with all civilizations -> there will be no space left for uninhabited planets. If you want to play with all the races (both the main races and the minor races) and you want colonisable planets, I'd suggest selecting mapsize huge or higher. This should do the trick.