The brightness keys result in the little brightness panel appearing on screen (see image) but the brightness doesn’t alter. I am running Plasma on a ThinkPad X395. I dual boot with Kubuntu and the brightness works okay. I’ve tried changing the brightness in Settings > Energy Saving > On AC but that doesn’t change anything either.
Any suggestions for possible solutions would be lovely! 
Don’t know the answer myself, but I tweaked the title of your question to describe your issue more closely. Hope this will help others understand your issue better so they can help you out.
I’d suggest trying xbacklight and see what it does.
I had this issue as well… if I recall correctly, it required changing/setting the kernel command line option for acpi_backlight in the grub.cfg file.
Try video, vendor, or native to see if one of those work first. There also may be vendor-specific drivers that you need to install… I didn’t for my case.
Thanks for your suggestions, I did do a bit of tinkering and never managed to solve the problem. I had reason to do a clean install recently and the brightness keys worked immediately. Thanks to everyone who commented with their suggestions. I know don’t have to watch films in the dark at max brightness, my eyes are saved! 
