(1) Rubber band used in the cell holder may not be tight enough to keep the two cells in contact with each other.
(2) Cell power has been exhausted.
(3) Maybe the two cells are not connected properly. The negative terminal of one cell must be connected to the positive terminal of the other cell.
(4) The connecting wire may be loose.
(5) The electric cell may be used up.
(6) Switch may not be functioning well.