Unable to adjust the brightness (only after sleep / wake up) of my Thinkpad / Windows 7 laptop

Usually I change the brightness of my laptop with:

Fn + Home / / The end: Increase / decrease screen

and it works well

However, ~ 50% of the time, after a sleep / resume, brightness is stuck at current level and I can no longer adjust it with the Fn+ … keyboard shortcut.

Also, adjusting the brightness slider here has no effect (it generally works!):

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The only solution is to restart the computer, and then after this it is possible to change the brightness.

Why did this happened? How to solve this little problem?