Giardia is believed to encrust in cysts and break again when stressed. Its also hard to find in a stool sample even a fresh one because it has to be shedding at the time the sample is taken. Did they do the elisa test?
Giardia canis in Dogs Dog Diarreha, Vomiting and Weight Loss | Giardia Protozoa in your Dog | GREATDANELADY.COM ACVC 2001 - Diarrhea Caused by Giardia and Clostridium Perfringens Enterotoxicosis
Make sure you pick up any waste even if it means loosing a shovel full of your yard to get it all.