I'm running Sierra 10.12.6 on a 2010 17" MBP. My "Show Desktop" no longer functions, no matter what key combo I have selected in Mission Control, or whether or not I use fn, or have fn checked in the keyboard control panel. All the other shortcuts in Mission Control work fine. This makes no sense, it all works right for other users on the same machine. How can I fix MY OWN user prefs?
Tried trashing user pref com.apple.symbolichotkeys.plist, that did no good. Finally used a scattergun approach which lost all the shortcuts in my Dock, but that's no big deal, it's easily repopulated. Here are the ~Library/Preferences that I trashed: com.apple.dashboard.plist com.apple.dashboard.plist.lockfile com.apple.desktop.plist com.apple.desktop.plist.lockfile com.apple.dock.plist com.apple.dock.plist.lockfile
Replacing the com.apple.symbolichotkeys.plist may or may not be necessary as well. Then reboot, or log out and back in. Voila- everything reset to defaults, and it works for the first time in months.