If the upright piano hammer isn't striking the strings

[maxim_tuner]

https://youtu.be/dW6KsxycbiI
If the upright piano hammer isn't striking the strings, it may not be due to 'the capstan of the key' being too low, as the entire 'wippen' would need to be low along with 'the jack' too there. But we don't see it's there. It's possible that:
1'The jack flange' is broken.
2 The pin/screw securing 'the jack to the wippen' has come loose or fallen out.
3 'The jack flange' has detached from 'the wippen'.
4 'The jack spring' has come loose, is damaged, or lost. The spring should push the jack forward.
How to Fix It:
1 If the pin/screw securing 'the jack to the wippen' has come loose or fallen out, you can attempt to reinsert the pin into 'the jack flange's hole' using a screwdriver.
2 If 'the jack spring' has come loose, is damaged, or lost, gently pull the spring upwards to increase its tension. If it's damaged, replace it with a new one.
3 If 'the jack flange' has detached from 'the wippen', you'll need to remove the action from the piano and unscrew 'the wippen' from the action. Then, use PVA glue to reattach 'the jack flange' into the groove of 'the wippen'.
4 You have the option to either glue the broken flange or replace it with a new one.
More full instruction how use (3-4) gluing 'the jack' to 'the wippen' about
THE Instruction for repairing a loose jack flange:
1. Remove the action from the piano. Detach any pedal rods from the action. Locate the screws or nuts holding the action brackets to the piano’s plate. Remove these fasteners, tip the top of the action toward you, and lift the action out of the piano up by grasping the hammer rest rail and lifting.
2. Locate the whippen that has the loose jack flange.
3. Disconnect that note’s bridle strap from the bridle wire on the whippen.
4. From the back of the action, remove the whippen flange screw and remove the whippen.
5. Remove any loose glue of or other debris from the jack flange and whippen check for a dry fit. Apply one drop of PVA glue to the whippen where the jack flange will be attached. Be sure the jack flange is properly aligned, 90 degrees to the whippen, and there is sufficient glue as indicated by some squeezing out. Let glue set a few minutes and remove any excess.
6. Reinstalling the whippen can be a challenge. Be sure the jack spring is seated in the whippen and jack. Pivot the jack against the spring enough to position it against the hammer butt leather as the whippen flange is also being positioned against the whippen rail. It is often necessary to have the whippen flange screw already in the flange due to interference with the damper spoon. Attach the whippen flange to the rail with the screw.
7. Keep the whippen raised in the normal position until the bridle strap is reattached to the bridle wire or the jack may jamb on the hammer. On many pianos the bridle strap threads through a hole on the hammer butt back stop.
8. Check for proper operation of the repaired note.
9. Replace the action in the piano. There are a number of studs in the piano that the action brackets rest on – one stud for each bracket. Handling the action by the hammer rest rail and tipping the top of the action toward you, position the action brackets onto the studs. Often it is easiest to judge when the action is properly aligned by observing how the whippens will rest on the capstans. When the action brackets are resting on the studs, tip the top of the action you toward the piano’s plate. Attach the action to the plate with the screws or bolts. Reinstall or re-position any pedal rods.