I had this problem with the row "z" to "m" and a few more around. They stopped working from time to time and then recovered. Then once I noticed they had stopped working after the MacBook was on a table but supported only in the centre of its base. Hence I wondered if the compression in the centre was somehow harming the contacts of the keyboard. So I decided to give it a stretch! I put my thumb in the middle of the row (between the keys) and the other fingers on the left and right borders for the macbook and I pressed with my thumbs. And voilà! It worked!!! Sometimes the keys start to fail again and I repeat the exercise; it has worked all the times! At least for now I have saved 500 euros in a new keyboard. I guess my backpack is the culprit, as it somehow unnoticeably bends my System!