The upright piano key, upon being pressed, delivers a weak strike of the hammer

[themaximillyan]

In the video, we can see that the jack fails to fully lift the hammer after the key (C) is pressed.
 

1 It is possible that (the wippen) spring of the jack, which is responsible for the jack's striking force on the hammer, has become loose. How can you fix it's? You need to slightly stretch this spring to increase its tension. OR change it's to new.

2 The center pin of the jack may be too tight there. How can you fix it's? Try gently wiggling the jack from side to side in this area to the jack is lightly loose there. OR Put down drop oil into bushes the center pin of the jack. (undesair variant).
3 The key's capstan (wood barrel) may be set high there too. How can you fix it's? Attempt to rotate the capstan clockwise by half a turn to down it's.

4 The felt on button of (the let-off) may have become worn down and stiff. You need to lower the let-off button, bringing it closer to the jack's arc. How can you fix it's? Rotate wire hook of (the let-off) clockwise by half a turn. OR need to 'fluff up' the button (let off) use your fingers there