doesn't work is a poor description of the problem, with that description we have no idea what your issue is.
How are you locking the 'session' ? and by session do you mean your local desktop ?
do you get an error ? is so what is the exact error ?
when you successfully lock the session what do you see ? screensaver ?
When it fails does the screen saver come on but you don't get pronmted for a password when you come back ?
need more detail.
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