The monthly dewormer is an essential to good health. Some worms can lie dormant in soft tissue and will not be killed by the monthly. This can happen when mothers give birth and the worm can be passed to the puppy. The puppy may have picked up the worms from something outside like eating droppings from another animal. If she had been treated for worms just before you got her they might not show up on a fecal, but if there are worms present it will likely take several treatments to get them all. It sounds like the Interceptor is working like it should.
