Just guessing, but maybe a cold solder joint? Wire broken inside the insulation? Brush jammed in its well and not making contact with the commutator?? These motors are stone-simple, E-unit is the #1 source of problems. If you bypass the E-unit it should almost always run.
Off-topic, but the relay-based drop-in replacement for steam loco e-units is one product that was discontinued before its time :-(