Some people can have so many worms that it impacts their intestines.
I would look for an environmental source of infection, such as a pet or farm animals. Also, unless a whole family is treated at once, they can re-infect each other. Even clothes that are not washed in very hot water can be a source if contamination. So can the work environment if there is another person at work who is infected.
Using augmentation footpads and paddles concurrently, with the zapper can significantly improve zapping results, but only if the zapper is strong enough to support them.