Why Does My "Show Desktop" Mouse Shortcut No Longer Work?

For years, I've assigned the expose/mission control command "Show Desktop" to the fourth button on my mouse. All of a sudden, quite recently, the command has stopped working. I know it's not the button because reassiging a different command to the same button works, and assigning "show desktop" to another button fails to succeed. It's not the mouse, because I've tried two other different mice.

I've searched for answers to this, and the general conclusion is to delete the "com.apple.symbolichotkeys.plist" file. I've done this, and the strange thing is that after restarting the computer the command starts to work again, but not for long. It stops working at a seemingly random time. I haven't figured out if anything triggers this.

This is really frustrating because command works flawlessly with my workflow, especally while video editing, and it is starting to become a real pain.

So, for the record:

The trackpad gesture to "show desktop" works.

The keyboard shorcut F11 to "show desktop" works.

Reassigning "show desktop" to any other mouse button DOES NOT work.

Other commands on the same mouse button DO work.

Any help would be appreciated,

Thanks.

P