Why is the keyboard malfunctioning?

The next most common problem with the power would be the physical button being either broke or stuck. In order to repair the button(s) remove the faceplate of the device and make sure there is nothing interfering the motion of the button(s) then reinstall the faceplate and test the problem button(s). If the faceplate has been removed and the inside of the calculator is clean then the next most common solution would be the replacement of the keyboard.