Upright piano Zimmermann 1965, A hammer badly strike to a string.

[themaximillyan]

The tall capstan may have been unscrewed during operation or it's was bad  screwed in  to key of a uprigth piano. As a result, with a weak press on the "D3" key , this tall capstan did not allow high-quality sending of the hammer to the string, since the jack quick jumped off the hammer butt buckskin (hammer heel). The hammer did not reach the string at all there.
On the advice of Max, the piano lady owner's husband pulled out the key from piano and slightly half a turn, screwed the pilot into it. Now you can see that the "D3" hammer has become "live". A simple operation that any layman can do. In some cases, even a woman can do it if she can take her risks, I suppose.
Love your pianos, ask Max always, he will be able to give advice according to his own knowledge. And, you can, without calling the tuner, correct the situation yourself there.
Delight to us with yours exquisite sound, our Indonesian "Zimmermann"