I more cases than not, the cause of a button "sticking" is either the button is "jamming" against the hole, or the hole through the button block is swelling closed, causing the wood to grab the metal rod that goes through the keyboard.
If it's the later, you really need to let a builder do the repair.
Jude

Hi Jude,

Thanks for the insight on that.

I have had the first problem, that of the button sticking in the hole of the keyboard. I fixed it by very carefully enlarging the hole just a tiny amount using an x-acto knife. That was the suggestion of the builder, and it worked.

I didn't know about the second type of problem you described.

-David

It doesn't happen very often, but I have seen it.
happen. I remember it once happened that a friend of mine was playing on a hot summer night, and a drop of sweat fell onto the keyboard, and almost immediately,
one of the valves stuck open. I have some other "war stories" but I think you get the idea.
Jude