Q: iMac wired mouse not tracking up

Hello everyone,

I have a two-year-old iMac with OS X 10.9.5.  I have a wired mouse, not the bluetooth mouse. For some reason that I cannot discover, using the little grey ball allows me to scroll down pages but not up. So I have to go to the side of the window and move the grey sidebar up. It's a drag.
I may have changed the settings for my wired mouse, and that could be causing the trouble.   After changing all the settings in different ways, I still can't fix it.  I have cleaned the grey ball (upside down, rub around on clean paper), and cleaning the gunk off the track frame.  Below are the current settings.

Can someone tell me what I should try next?  I would really appreciate it.  Thanks!