I feel like this has happened several times now. I'll use the keyboard shortcut to take an image capture of the screen. Voila automatically opens but then nothing happens. The capture never appears in the library. If I switch back to what I was capturing and do it again, the image than appears.
Voila is running from the menu bar on start, so it seems like the delay in launching the app is causing it to forget what it is supposed to be doing.