I am thinking that if keyboard is of any significant age then - time for replacement ...... something from newegg.com or techforless.com perhaps.
Usual reason for sticking is spillage of something stickly but if not that and something that'll clean up, then may not be too much else you can do. Hard to tell entirely without the problem in front of me to look at - tho I have over the years managed to deal with key problems successfully a time or two.