whereas some are stapled.
from time to time, due to different humidity exposed, hammer head felt may come loose.
if not noticed visibly, the affected key tone will turn mellow or even will produce no sound at all.
attaching back is one of the way to solve the problem.
by using the traditional way of fixing the loosed hammer felt, thread is a very
useful tool. of course and a proper glue.
with proper glue and the support of thread will solve the problem.