The common reason for a dead tooth is a fine crack in the tooth, but
it can also be caused by a loose tuning weight.  Of course if the tooth
is a replacement then it may not be secured properly, or may not be
hardened and tempered properly.

Sometimes a tooth will not ring if it is tuned substantially higher or
lower than an adjacent tooth of the same note.  Two teeth tuned together
will sustain only if they are tuned no more than a few cents from each
other.

Robin Biggins
So. California