I thinjk there may be multiple cases where the turn key stays hidden when it should show up.
I found one case just now. If you colonize a planet and have the planet view up and assign social projects to tiles, when you want to dismiss the planet view, be sure to hit the done button on the planet view, not the turn button on the main screen, (they are right next to each other vertically, which leads to confusion). If you click the wrong button by mistake the turn button goes away and does not return, but all the other functions (as far as I tested) seem to work. Pressing the enter button gets you to the next turn and the turn button shows up again.
And a request: Please make the planet view window extend all the way to the bottom of the main window, thus hiding the turn button. (If it is important, I am in windowed mode and using a custom resolution of 1300*1140.)