Why can't I build the last SS Thruster?

So, I have all my spaceship parts built, except I'm missing one thruster. But I can't build another one - the thruster shows up in the build choices, but it says:

"SS Thrusters (Team Project: 0 left)
Required for Space Race Victory (2/3 Total)"

How am I supposed to build the last one? Why does it say "0 left", if I've only built two?

Are you sure it's not in some city's build queue already?