A 30 SECOND SOLUTION - NO SENDING TO APPLE!
PROBLEM:
I've gone through all of the solutions I can find online and believe I have the answer! My keys have been the same for months; it's been driving me insane. My space bar requires all the power my thumb can give it, causing me to habitually type destructively on other keyboards!
Apple's compressed air solution didn't work. Taking off keys and cleaning underneath didn't work. I took it into an Apple repair centre but they wanted to send it off, and I was about to return home to Africa (and didn't trust them) so that didn't work either. But THIS did work and, after six months of suffering these ****** keys, I'm now typing at high speed again on a perfect as-new keyboard!
SOLUTION:
I believe there is a build up of dirt on the minute EDGE OF THE METAL PART surrounding the keys, which effectively narrows the gap between the MacBook 12" body and the keys themselves, causing them to stick. It's too small to see, but it's there.
Apple's compressed air wasn't strong enough to blow the dirt buildup away BUT this is: insert a fingernail into the gap between the key and the case and slide up and down repeatedly to scrape off the build up of dirt on each of the four edges facing the key. Make sure the top of your fingernail is facing into centre of the key. Do this on each side of the key (so four slides on each edge of each sticky key). The result on each of my sticky keys is INSTANT. ALL THIS SUFFERING FOR NOTHING!
[Cue someone clever to make millions from selling a thin plastic fingernail-like scraper!!!]
Good luck and let me know if it works!!