Hi, I had an intermittent A key on my Macbook Pro Late 2011. Sometimes additional pressure made it work, sometimes simply nothing. I was already considering to replace the keyboard with a cheap one from eBay but before, I tried swapping the ribbon system taken from another standard key, doing deep cleaning with rubber alcohol and a lot of compressed air and hoover to be sure not even a residual debris was in there.
Nothing changed. At the end, after a very close inspection, I found out that the small silicone cup has a sort of liquid drop into it. I was able to see a little bubble while pressing. So, having the ribbon removed (again) and keeping the cap in place without removing it (i didn't want to risk to break it - there was a lot of stock superglue keeping it in place- I raised on side with a needle an I have dried it with hot hair from a small hair dryer, being careful to don't get it too close or too hot. I spilled no liquid into it, but a low amount of cleaning product could have leaked in there.
Anyhow, issue fixed.