carefully take off the keycap and clean out underneath the key and check that the plastic scissors that hold the keycap is in correctly and has a bit of a "springing" action (if not, the scissors are probably broken). I would be highly surprised if the actual button itself is sticking, as they are just soft keys basically. More likely the scissors are broken or have came loose, or there is dirt/gunk underneath the keycap causing it to stick (which seems likely since they are all sticking in the same general area).
If you take the keycaps off, gently pull up from the top of the key (side closest to screen) and pop it off carefully.