The two wires that come from the tender attach directly to the trucks - one wire to each truck. If you have an ohmmeter, check for continuity, one wire to one truck and the other wire to the other truck.
If either wire is open, take the tender shell off and fix it.
If both wires show continuity to their respective trucks, see how the tender wheels are configured.
One truck will have the metal wheels on the left and the other truck will have the metal wheels on the right.
If there are no metal wheels on one or the other truck, that is a problem.
If all the metal wheels are on the same side, ie, touching the same rail, that is a problem.