Scroll bars in OS X Lion are hidden until you begin to scroll using the trackpad, mouse or keyboard. This default behavior works well with the trackpad by making windows look cleaner. It might not work for some of you especially if you use the mouse for dragging the scroll bars. Luckily it is simple to change this behavior from System Preferences.
Under System Preferences>General, look for the section “Show scroll bars”. Select the radio button next to “Always” to keep the scroll bars visible at all time under OS X Lion.