Keyboard shortcut issue

Whenever I use the Ctrl + NUMPAD6, nothing usually happen.  However Ctrl + NUMPAD8 and Ctrl + NUMPAD4 works all the time.  Normally I use these shortcuts to go from 12 - 14 pt, or 14 - 18 pt. I checked Options/Customization/Commands and the shortcut is active.  Any ideas why it isn't working?