Piano work often feels like detective work when trying to figure out why something doesn't work right. One symptom can have multiple causes. I've recently finished putting a new set of hammers on a piano this last week and was surprised to see odd irregularities in my hair line. Turns out some of the jacks had been reglued a bit funny (see picture): the jack, which propels the hammer towards the string when the key is pressed, sits in the middle of the whippen pictured. On old pianos they can become disconnected/unglued. Normally, they sit perpendicular to the whippen. This one had been glued, but at quite an angle!
-- Piano Technician Bryan Hutchison