It's hard to be sure what's going on without seeing your specific configuration, but as Dune indicates, you can't simply set an object to "Member of a population" to create a population. You need a Populator node with the object inside it and set to "Member of a population". The best and easiest way to set it up right is just to create a Population from the beginning.
If that doesn't clear things up, perhaps you can post a .tgd or screenshot of your node network and settings.
- Oshyan