Has anyone who is using the Dexcom been experiencing a lot of sensor failures? In the last two months about 3 out of 4 sensors have failed before the 7 days. Dexcom keeps sending replacement sensors but can't explain the problem. They said other people are having the problem too. I getting really fustrated with constantly having to call customer service. I did own stock in Dexcom but I sold yesterday as I'm feeling a little unsure about this product lately.