Worn buckskin on the hammer butts (this is rare in the bass section)
can cause this problem.  The most likely cause is the backchecks are not
catching, due to wear or mis-adjustment, or due to lost motion between
the piano action and player action, or any reason that causes the
player action to not drive the bass hammers through a complete cycle.

The bass hammers are heavier and tend more to "blubber" if the backchecks
are not working.

Hope this helps.
Andy Taylor