Set it with the buttons at the top and spray or drip some penetrating oil (I like Kroil, but there are other good brands) into the area where the buttons are. Let is sit awhile (few hours) and test the buttons. Repeat a few times, they will usually free up. Do not force them if they are not ready.
Next alternative is to remove the handle and plastic top so you can get penetrating directly into the button mechanism.
But the Kroil has always worked out for me.
John Sullivan, January 2016