Why does the hammer of the upright piano not strike the strings due to weak pressing to the key?

[maxim_tuner]

https://youtu.be/W672nbXlnCo?si=MLlTyVsSpeG8jSJ4

• On most uprights, the hammer only goes to about 3 mm from the string while it is pushed by the jack. The let-off buttons are used to adjust that. The easiest way to do that is to get some strong magnets, and use them to hold a piece of wood 3 mm thick against the strings, and then adjust the let-off buttons so the hammers just touch the wood before they fall back. You can use thinner wood, but it depends on the tolerances of the action, and the hammer blow, lost motion (capstans), and aftertouch will be affected.
  
  If the key pushed the hammer all the way to the string without releasing, the hammer would stay on the string, keeping it from sounding. (the action can be adjusted to make that happen)
  
  Usually the action of home pianos are adjusted with more of a margin of safety - so pressing the key will bring the hammer only close to the string before releasing. The amount is adjustable. Closer brings more control, but environmental changes can bring it too close and keep the action from working properly. Too far away makes it easier to press the key slowly and not have the hammer strike the string at all.
  
  Yes, 'the hammer' realy doesn't move BACK rightly as if it has bad condition 'the hammer spring' (it's broken) or it's very tighted 'the hammer' into 'the hammer flange' there. Because 'the back check' as me understanding to working sharp rightly there.
  
  I think that 2 cause there: Or high 'the capstan of key' or tight drive of the hammer there.