That was exactly it. The tooltip says "Construction points +2" - not the most intuitive thing in the world, but now that I know what it means, all of the mysteries of the universe are solved! :) Err... until I stumble onto the next one. Thank you both. :)
I've searched the Internet and can't find any reference in the wiki or the manual to this situation, or anywhere where google seems to have indexed. Sometimes a starbase module will show up as available, but will be greyed out once I have a constructor there and won't build. Orbital Institute seems to have this happen a lot. I don't see anything in the tooltip indicating why it can't be built. Does anyone know if this is