I'd be inclined to suggest checking out ifixit.com etc. for your Mac and its keyboard and how to pop off the key covers _ CAREFULLY!!!, and then use some 99.9% isopropyl alcohol with some q-tips and/or toothpicks and see if you can't clean up what's causing them to stick.
I'd avoid the sites that might suggest removing the keyboard and putting it through your dishwasher's cycle!! At least until no other fixes work!!
PS: Depending on your eyesight, you may want to use a pair or model or jewelers magnifying glasses or headband as it doesn't take much to miss what might be causing the sticking. And it could even be a broken plastic spring piece.
Good luck …. ;D