I find that when I use some programs (usually that heavy graphics programs such as games), the tooltips for icons in the System Tray (notification area where the time and volume controls are displayed) will appear behind the taskbar instead of on top.

For quite some time I have just rebooted the computer to get those tooltips back.   I have found that on my system, I can right:

* Right click on the task bar and select "Properties"

* Uncheck the option "Keep the taskbar on top of other windows".

* Hit the "Apply" button

* Enable the check on the option  "Keep the taskbar on top of other windows"

* Hit the Ok button

That usually restores the tooltips to appear in front of the taskbar instead of behind it.