Check the power options first, could be stuck in portable mode when the charger is plugged in.

Most laptops have some sort of integration between the quick key software and screen brightness.
Could possibly be an issue with some software/drivers.
If you have the driver & application cd/dvd chuck that in and reinstall any quick key software.

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2015 Forester XT Premium
2020 VW Tiguan Allspace
teejay: If I wanted a comeback Id wank in front of a fan