Dedicated buttons rely on software supplied with the board. Check that you do have a disk for your brand of board. If you have go to Control panel/ Add/delete software find the program [ Logitech lets say as an example] delete it and re install
Or you cxan go to Start/programs/ look for it/ right click on the icon and see if a dedicated uninstall comes up. Then re install.
There is another possibility If this button is used a lot and the mechanism such as it is has become distorted. If you can take the board apart you will find The mechanism is usually infact a plastic membrane with contacts over each button. Locate the offending item and manipulate it back to the same profile as the remainder
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