At first I thought this was a minor bug, but it turns out to be a constant thing.

Basically, the Iconians and Torians only build improvements on roughly half their planets.  A large number of worlds just sit barren, except for the initial colony.  They will upgrade tiles once things like Soil Enhancement or Terraforming is researched, but they won't build any structures.

It appears to be ONLY the Iconians or Torians that do this.  Every other race builds improvements correctly.

I am positive that it has nothing to do with lack of money, logistics, population, or anything like that.  They simply will not build on half their controlled worlds.  When I play, I do so on gigantic maps with lots of habitable planets, so I don't know if this would occur on small maps where they have like 5 planets.