1)I posted a study abstract here a while back that found that there were potency issues with some generic mebendazole products, so hopefully you're taking Vermox made by Janssen.
2) Sometimes what people think are roundworms are actually tapeworm. In this case, some praziquantel or perhaps niclosamide could be considered.
3) In some cases, worms are residing in the upper portion of the small intestine. After being abruptly killed off they may pass through partly digested. However, worms dying on their own are more likely to die slowly and pass out whole.
4) Sometimes there are only one to a few worms present to begin with.