Why may galvanic cells go dead?
Galvanic cells go "dead" for several reasons.
One reason may be that the electrode which is the anode (being oxidized) may simply be used up, i.e. there are no more atoms to remove electrons from. In this case, there are no more electrons moving.
A second reason is that the substance in the salt bridge or your electrolyte may have a level too low to allow for movement of ions.
A third reason that a galvanic cell may go dead is that the waste products may accumulate to such a degree that charges can no longer move.
Have a great day!!