When playing any game on Windows, if I trying to remap any key on my mouse that is higher then button 5, it won't notice it being pushed. I doubt it's a problem with the mouse, because the software that comes with the mouse can detect the other buttons when pressed. I have a Corsair Vengeance M90 Black 15 Buttons.
Not all games support these extra mouse buttons. It depends on what your mouse is actually sending as a command to your computer. If they aren't picking up in game though, you can most likely use the software that came with the mouse and rebind them to something else. Maybe some keys you don't use like the arrow keys?