Removal and insertion of a single key

Try removing the "problem" key and re-inserting it. To remove, place a small screwdriver under the front edge of the key and pry upward lightly. Be careful not to flip the key across the room when it comes free.  

To install a key, rock the keyboard front to back so the spring pivots freely into the center of the chimney area. It doesn't have to be perfectly centered, just not touching any of the sides. At this point, place the stem of the key into the chimney with the spring riding up into the center of the stem. Press down until it snaps into place. Actuate the button and feel the tactile switch. If you don't feel the snap, remove the key and do it again.