You can use something like a paper clip and move it around the bezel of the display until you feel the tug of the magnet. Once you identify that point then you can see where on the computer lines up with the magnet.
It is possible that the magnet has moved out of position and is not lining up with the switch.
You should check the Power Options in Control Panel to see the settings for "Choose what closing the lid does". You can also check to see if Sleep or Hibernate modes are enabled. This setting is more likely to be the problem than a failed Hall effect sensor switch.