how to fix a dead "down" key

I have a iMac early 2008 version. After working hard for 5 years, the "down" arrow key is dead. This is what I have done so far.

1) Remove the key cap and the associated plastic assembly

2) See a "white bouncy dome" under the key.

2) Open the "Keyboard viewer".

3) Click the "down key" on the "Keyboard viewer" ==> it works fine.

4) However, when I press that "white bouncy dome", no reponding.

5) use the compressed air to clean the areas. still no responding from this key.

What should I do next to fix this dead key? Search many forums or youtube videos, but all I got is how to "remove" and "install" key caps. I could not find a proper one to solve my problem. Thanks a lot!