stacks won't collapse

[timoteo]

Suddenly, my stacks won't collapse. Files that indicate that they are stacked in the Viewer won't collapse now. I can see all files in the stack displayed in the viewer and the topmost file in the stack shows the number of files in the stack but when I click on the "number of files in the stack" indicator, the stack won't collapse. I tried selecting all files and doing right-click "Stacking->Collapse" but nothing happens.
Is there some key combination that causes this?

[Mario]

Have you enabled the File Window option to always show stacked files?

See "Show stacked files" in The File Window Toolbar

[timoteo]

That was it. Thank you.