ship building problem

In testing supernova I of course tried building ships asap.  This seems to be a bit tricksy in my hands.  My first ships didn't show up in the ship building list;  they did later so I suspected perhaps a turn had to go by before they showed up.  Today I went further and picked up the battle cruiser tech, then tried to build a new ship medium sized.   I could not get the game to take my new ship and I pondered that maybe I didn't have some prereq tech.   But if the hull type shows up in the building list, it should be buidable no?  Huge likelihood that I am just doing something wrong.

Doing some more testing I will try to be more specific.