I have had multiple games in which I was unable to construct the Orkulus starbase. This has happened in two different situations.
1) I heard a message saying that the gravity well was too small or something to that effect. This has occurred in the case of a gravity well containing a central asteroid with some surrounding dead asteroids in the same well. This has also occurred in what appeared to be a normal gravity well with a volcanic planet.
2) I heard a message saying that there was another structure in the way, when in fact there were no structures at all on that side of the planet. I think my planet in that case may have been an asteroid.
This problem of being unable to construct a starbase, especially at a key choke point, can create a major disadvantage and swing the tide of a game. As an aside, the above situations all arose when attempting to build a starbase the normal way using the planetary menu. I don't think I tested to see whether I might be able to use the starbase ship item to get around the problem. Even if that would work though, the extra constraints involved in doing that would still put the affected player at a disadvantage.