It doesn't happen often with normal playing but I can often force it to happen by playing the note normally and then instead of releasing the key, holding the lip at the front of the key and jiggling it quickly up and down while slowly raising it, lowering it, and raising it again. The hammer slips out of checking back towards normal rest position, but the jack stays tripped, working its way up the back of the hammer butt.
Yes, that's the behaviour, although a bit more easily triggered. My grand-daughter (who is a beginner) plays very lightly and tentatively and if she doesn't quite hold down a note in the left hand firmly enough before repeating it, the key can sometimes hang. With my adult hands and arm weight I don't see it as much but I occasionally get it.
Could you be more specific? What did you measure?
Key height, blow distance, let off, key dip and checking distance. All are correct and consistent except checking distance, these are all over the place. After a bit of experimenting I have found that keys where the check distance has been set way in close (under 10mm) do not have the problem. The keys where the check distance is out at the recommended 15mm (from Randy Potter's book) are the only ones that have the problem. I wonder if a previous tech has bent the back-checks in to fix the problem when it's been reported by the previous owner.
I guess I could simply reduce the checking distance on the problem keys but I suspect this would just be covering up whatever the real problem is.
I haven't really measured aftertouch, I guess I don't fully understand its role.