The hammer of the key (F4) of the upright piano has weak hit to the string every other time. How do fix key's hammer the

[maxim_tuner]

https://youtu.be/sbEQAgw5r3M

• There are several reasons don't we hearing sound there: The reason is not in the key there, but in the hammer I'm think. But I'm inclined to believe that the reason is either that the 'the wippen of the jack' is not active there, or 'the hammer butt' of the hammer does not go back fast. May be the jack spring broken either or it's get out there. The capstan on the key very tall there.
  How fix it yourself:
  1 Remove the front panel of the upright piano and carefully see as the hammer works in the action there.
  2 If 'the wippen of the jack' is not active that cause there.
  3 If the jack spring broken. Need to set new spring there.
  4 If the jack spring got out from it's hole. Return back it's.
  5 If the hammer does not go back fast that reason on 'the hammer butt' there. Need remove the hammer from the action of the upright piano then there. And see on the hammer. Why it's bad works there? If you see that the pin of the hammer butt flange' get out then:
  1 Once removed hammer, use the pliers there. Open out the cheeks of the pliers there. Pressing on one of the cheek of the pliers on the tip of the pin but second of the cheek set to the 'the hammer butt flange' and carefully adjust it back into need position. Be cautious not to over tighten or damage the pin and the hammer there.
  2 More often the hammer has a bad stroke there too.
  Need to loosen (shaking side by side) 'the hammer butt flange' where it's connecting with the hammer there. If it does not work well, then drip lube on 'the bushings' of 'the hammer butt flange'
  3 The skin glued off from of 'the hammer butt'. Need to glue on there.
  4 Seldom the tall capstan of a piano there. It may have been unscrewed during operation or it's was bad screwed in to key of an upright piano. To try screwing the barrel of the capstan into the key.
  5 Very seldom reason when 'the wippen flange' was very tighten. Weak it as in (2).
  6 'The let off' of the upright piano was unscrewed out more than need tall up there. Screw 'the let off' to back.