Have you checked the terminal pins and the contacts inside the cord for rust and/or corrosion? Have you tried it with another cord? Do the terminal pins seem loose? Have you looked inside the table stove to see if all of the connections are tight and no wires are broken? With stranded wires, you can have some wires broken which will affect performance. Have you checked to see if the magnet in the center of the burner is rising all of the way? That could be balky after 80 years; inhibited in its movement by corrosion, rust or shmutz.

There is only one element and it is operating at less than full wattage so we need to check to see what is preventing full current/voltage from making it to the element.  It could be either end of the cord not making good contact with what it is supposed to contact. It could be the switch not moving as it should. It could be a wire on one side of the switch or the other not securely connected. It could be a wiring connection at the magnet. Clearly, you are going to have to go inside the table stove to look for the trouble. Best of luck.

Page 14 of the January, 1948 issue of Consumer Reports rates automatic coffee makers. Your General Electric 129P18 was among the 6 tested. Here, I think is the kicker from page 16, "Coffee kept at constant temperature by thermostat." I would be willing to bet that the stuck thermostat is the problem, keeping the stove at the "keep warm" setting instead of  allowing the high heat setting to begin the brewing cycle.