>- I don't understand is why it took almost three weeks for the zapper to start killing them,
A couple of points here:
If you are not using augmentation footpads, then you would probably experience faster and better results if you did. Many zappers are not strong enough to fully utilize this option.
Second, many organisms are not killed directly by zapping. Certain worms require symbiotic microbes in their gut and start dying after the microbes are dead. For that reason, these worms may take a couple of weeks to die (probably from starvation or digestive distress).