Product quality issue

I am writing to raise a concern regarding a NexGard product I recently purchased from Takealot. I usually buy NexGard directly from my local veterinarian, however on this occasion I chose to purchase it from Takealot as it was more convenient and saved me a trip to the vet. The product was administered correctly and initially appeared to work. However, it has only been five days since administration and I am already noticing fleas on my dog again. Based on my prior experience and product information, NexGard is meant to provide protection for up to 30 days. This has led me to believe that the product I received may be defective or compromised. I am extremely disappointed, as this is not the level of quality or effectiveness I have previously experienced with this product. I would appreciate it if you could please investigate this matter and advise on the next steps,