Basically it means you have an open circuit in right motor power supply. In most cases it happens between the brushes and the commutator. That's a common thing if the motors are neglected. Brush carbon dust has to be blown every once in a while otherwise it creates short circuits, burns and soot and makes the commutator go bad. Unfortunately once when these motors go bad they can't be fixed. You have to replace them with new type NEO motors. Search for info on this forum. You can machine the commutator and change the brushes but that will just postpone the change.