Worms in child's stool but adhesive strip test negative?

How can that be.

Child (2) had live, small white worms in his stool. I went to the doctor in a few days for worming medication and an adhesive strip test, which was negative.

They are threadworms because a child in her kindergarten group had some a few months ago.

Since the first dose of the medication, there are no more in the stool.

How is that possible? I did the test in the morning, hadn't washed beforehand, and she hadn't been to the bathroom.

How can the test be negative if there were worms in the stool?