Used for my puppy with suggested amount, went to vet a week later and he had roundworms.

Jodi, read the label. You have to give it multiple times so that you catch the worms when they are susceptible to the wormers. You can't just worm them once and have everything gone. It only kills the adult worms, so you have to keep giving it every week or two (per the label) so that when the eggs hatch and the worms mature, it can kill them. The vet is only seeing the eggs, which would have still been in the feces if you had just given it to your dog a week before.