hammer "throw" on hard blow

[David Nereson]

    I often encounter, when using test blows, hammers that get "thrown"
left or right on a hard blow, missing one of the strings of the unison,
even though they hit all three on a soft blow.  This can happen just as
often on straight-bored hammers as angle-bored ones.   I'm not sure
why.  You can move the whole flange/butt assembly left or right so all
three strings will be hit on the hard blow, but then on a soft blow, it
barely contacts one of the outside strings.  Does the flange twist or is
the jack pushing one side of the butt (or knuckle) harder than the other
or what?
    A similar situation is that of the "rubber shank."  But usually
"rubber shanks" wobble badly during and after the blow, like throwing a
knife into a tree, where after it hits, the handle wobbles laterally
(doing-oing-oing-oing-oing-oing...).  This hammer being "thrown,"
however, is mainly during the blow, and no wobble after impact with the
string.
    How to correct?
    --David Nereson, RPT