When I have more than 1 instance of an app the taskbar shows it by a line at the bottom. However, when I hover over the icon in the taskbar, it doesn't show both instances so I can choose which to open. In fact, even worse, when I click on it, nothing happens. It only works if I only have 1 instance open.

For example, I have 2 MS Excel files open. When I click on the icon in the taskbar nothing happens. Also, nothing comes up when I hover over it.
Anyone know how to fix that?