Middle Mouse Button Doesn't work

After I upgraded to 12.04 LTS my middle mouse button stopped working... it worked fine before the upgrade. I can scroll with the wheel but when i push it down it refuses to work. Can someone tell me how to make my mouse work again?

The same issue persists with Ubuntu 17.10 and Xubuntu 17.

I found this helpful, too:

https://askubuntu.com/a/902976/354678