I don´t know which specific type you´re referring to Brian, as i´ve seen most types of Wilesco generators mounted on a D32, and at least 2 that i´d consider dedicated D32 generators.
My initial suspicions would be:
Poor magnets
Wornout brushes.
Overlubrication (which will yield poor contact if the commutator gets oiled as well).
Burned out coils (since that they all can run as motors as well, but easily see to high voltage or load).
Does the dynamo run, when you serve it som low voltage?
If it rotates at 2-4 volts everything should be good inside electrically, but if you have to lift the voltage much higher to get it to turn over, weak magnets would be my suspect.