If you are using your FaderPort 8 in MCU mode (for DAWs like Ableton Live, Logic, and more) you may notice that using Solo Clear or Mute Clear only works on the group of eight channels currently focused by the FaderPort 8.
Some DAWs may allow the Solo or Mute Clear key to function for all tracks with the use of a modifier key.
For example:
-In Logic you can engage the "Shift" button on your FP8 then hit Solo Clear or Mute Clear to clear all tracks
-In ProTools you can hold down the Shift key on your QWERTY keyboard when pressing one of the clear buttons to clear solos or mutes on all tracks