Why is my brightness level not decreasing?

In my experience, this is often due to issues with your graphics driver. A simple check for a Windows driver update might help. Press the Windows Key and X. Select Device Manager from the menu that appears. Find the display adapters section and pop it open. You may even see it's open already if the issue has been detected by Windows.

Right click whatever your computer has listed as the Display adapter. Likely for you it will be an Intel Graphics device. Select Update Driver to see if a newer driver is suggested by Windows.