I started a new adventure campaign and at the 3rd stage, regardless of the amount of clicking I could not play it.
I created another adventure campaign and at the 9th stage I could not select it. Alt-tabbing seemed to help or it might have been switching from full screen to windowed (I'll try and repeat the steps if a stage becomes un-selectable again).
I found someone in the steam forum with the same problem.
"In adventure mode, i can't click on the fourth level..." "And i'm stuck...Anyone can help with this bug?"