In the key cap well, you will see the silicone rubber snap-action key switch actuator. When depressed, this brings two contacts beneath it together. This is glued in place and you will probably tear it if you try to remove it.

First, blow out the dust and loose dirt. Then, clean out the well with isopropyl alcohol and an old toothbrush. Pay particular attention to the metallic area around the silicone. Allow it to dry.