Key only works without keycap on

I have encountered this in the exact same scenario on a 2012 MBP model, switching from Arabic to ABC keys (The option key in my case was not working). I managed to solve it by removing the rubber dud, then re positioning it slightly and then clipping back the key cap. It worked after a couple of attempts. I cannot confirm it worked for the long haul as the unit went back to the owner, they haven't said it has stopped working after 6 months or so..... maybe worth a try if any other methods failed.