Suddenly can't expand Visual Tracks

I can expand Visual Tracks in Timeline so I can work on one slide at a time, but suddenly this no longer works in Storyboard. If I click on a stack then it opens, but all are highlighted. Until now I could click on the blue icon on the lower left corner or hit control-V to expand; now that doesn’t work and the menu item is dimmed out for Storyboard, but works for Timeline. I’ve closed the app and reopened but the problem persists. Ideas?

More info: I can expand Visual Tracks in a different slideshow but not in the one I’m working on now.

For those of you who might have this problem, I figured it out. I’m working now on a MacBook Air with a 13-inch screen. There’s only enough room to expand to four slides. Would be nice if this was documented! I’ve notified Support.