When a player piano bobbles its hammer, that means the hammer assembly
is bouncing between the jack and the string, and so the back check can't
catch it and hold it.  Sometimes on uprights, even when the jack is
sufficiently clear, a poorly regulated hammer will bobble between the
check and the string.

This is more of a problem on uprights because of the poor control afforded
the hammer by the check. There are several things you can do. First, you
can be sure you have taken out most of the lost motion between the player
strikers and the action whippens. You also have to regulate them by
adjusting the striker travel as well as the rest point of each striker.
Too much travel and the hammer blocks the string. Too little and it
bobbles.

If the hammer bobbles when the key is played by hand and when the player
plays it, the problem is in the action alone. If the hammer bobbles only
when the player plays it, then regulate that note to strike with the same
depth as the key does it, and it usually fixes the problem, but not
always! At that point, you may have to tip the back check forward on its
wire a little.

Bobbling hammers can be a really tricky problem to fix, and anybody who
has the problem also has my sympathy. It's very difficult, short of an
action restoration, to cure it completely on some pianos.

Craig Brougher