Solid checking is a balancing act between hammer blow, key dip and let off.
What I hear in the video is hammers that are send too fast.
(I just listened once or twice - it is good to make videos in that sort of case)
May be simply having them too angled at rest can create that (rest rail too thin)
Remember vertical actions are way more "efficient" than gran actions, the hammer is not retained by inertia.
A small dip on the buckskin leather may be enough to have the angle of the jack send the hammer extremely fast, faster than the normal motion of the jack. that create a loss of contact hence a loss of touch at FFF, but generally, on a vertical in any case playing strong means the hammer is immediately out of control.
Always control that the jack have a little space to move farther when the key is at full dip, this problem is rare but happens.
The catcher also can block the action somehow an then adding some key dip does nothing. you can have 11 mm key dip and the key stopping at 10m mm with some actions. the same if the jack limits the system (and very rarely the dampers).
Anyway if the butt have some trouble :
that can happen if the piano played for long with too much space between key an action for instance, or if the piano is fitted with a "silent" system and letoff is too much near the strings.
It will be impossible to allow for a small let off, unless the butt can be "bolstered" or the buckskin and undercloth changed.
Some otherwise good actions will need a large let off, some U1 for instance. if not checking will not be firm.
Testing with the backcheck wire bend is the most precise way to control the whole motions (and the final position of the jack vs the butt)
You will have other experiences like that. Before touching the regulation of the precedent tech, as he sol the piano, it may be up to him to correct regulations problems (guarantee ?)
If not the customer need to pay for a +- complete regulation work, anyway at last 3 parameters going together, you need to influence them all generally.
That is when you nee to understand well what goes on (pianistic touch helps , also) so to use shortcuts, as papering under the key frame, changing the hammer travel distance, and so on, so you need less time to make the action work.
Voicing help checking by helping hammer rebound, I find.
You also need to synchronize the key bottom with the tone production, that is what gives the utmost control on tone for the pianist.
So there may be you may hae nee to reduce key dip a a little.
PS it is extremely rare that too small key dip make the jack block the hammer, it only happen in extreme bad regulation, and do not create that sort of double strike , or only when playing lightly mostly bad checking at FFF playing and hard tone as the key make noise before the hammer make tone.
You are self employed now ? refrain please to be critical about what did the precedent techs, it may be more complicated to solve the issue than it looks at first sight.
No piano can be regulated with "standard distances" even if they help. one that matters is key dip around 10 mm and key level depending of manufacturer specifs. the rest is up to you.