As I said, I have Ubuntu 14.04. I bought a used mouse to use with my laptop, and when double click didn't work, I assumed it was the mouse at fault. So I just bought a brand new mouse. Double click does not work. The older mouse was a Logitech M310 and the new one is a Logitech M510. Any answers?
Let's start with the simplest answer possibility first: Go into System Settings>Mouse and Touchpad> and under "double click" try dragging the speed down to low and see if that helps. For some reason my Steel-Series mouse needed some adjusting unless I doubleclicked extremely fast.
Hope this helps.
@"Mike Uresti": this definitely was the cause, and setting the slider to Slow fixed it; thanks!– ccpizzaSep 17, 2015 at 12:52