Under the plastic key, there is small dom shaped soft rubber. Inside the dom, there is a pointy part which contacts the map(?) in the keyboard and registers that the key is being pressed. The problem you are having is that the pointy part is not having good contact with the map in the keyboard.

It could be the rubber dom is cracked or just out of place. If it is cracked, you can replace with other key's rubber dom that you don't usually use such as "Scroll Lock" or "Print Screen" or even "Pause" key. If it is just out of place, reposition it and that's it.

Forgot to mention that, you might need to actually cut out the rubber part in order to replace it. Use ruler and shape knife.

Good luck fixing it.