How to show app window When I change from one running application to another, I no longer see the topmost (or any) window from the new app. I now have to select a window from the menu bar. 

This is a new and unwelcome behavior. Until semi-recently, switching to another running app would bring up whatever window was last open. I suspect this is a setting somewhere, but I don't know what or where. Can someone suggest how to change this to the previous behavior? 

On MacBook OS X 10.8.2
I discovered that using command tab to switch to another running app, then using command ↑ (up arrow) displayed the window for the newly active app. To my surprise, I could then get an open window when I changed apps without having to repeat the command-up. I have no idea why this worked or how I got into the situation in the first place.