CX-22 Trak Ball - Repair?

I have a 2600 Trak-ball that is broken.  It works side to side, but the up/down motion does not work.  I've made sure I'm in "joystick" mode.  Also, I opened up the trak ball and cleaned it up, oiled the 5 little bearings, and reseated the chips.  Still no up/down motion.

Does anyone have any other ideas I can try to get this thing working?

Thanks.