When I press the Fn+up/down, the bar shows up and changes but the brightness of my screen doesn't change...
Could anyone tell me how to sovle this problem?
There are at least 5 threads devoted to this issue in the first 20 spots on this subforum: please read them and see if your situation applies to any of them.
Also, you mention your laptop model in your thread title, but tell us nothing about your hardware, which means you either expect us to search for it to determine the make of your graphics card etc, or you do not think it relevant. If the former, you are in for some disappointment; if the latter, you are incorrect.
Please read How To Ask Questions The Smart Way it will help the community to help you if you include important information and what you have already tried.