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Then try a pram reset, this resets all the system busses, it is the next most troublesome.
To do a PRAM reset, you need to restart your machine and hold down these 4 keys all together at the same time.
‘Option’, ‘Command (Cloverleaf or Apple)’, ‘R’ and 'P' all together at the same time.
Keep them down until you hear 4 startup chimes, this is the sound that your computer makes when it starts up, it is like a boonngg. The screen should also cycle from white to black and then back to white along with the chimes.
The sound should also get louder.
This resets all of the system buses on your computer, USB, FireWire, display, ethernet and your keyboard/mouse/trackpad.
This will correct many faults but not necessarily yours.
After doing that check System Preferences/Desk top to make sure your choice is indeed selected.
Select different ones and see if that helps.
Please let me know.