Dexcom sensors

My sensor 3 weeks ago had ???'s then failed. Then, last week's had ??? and was inaccurate (was too lazy to change it) but never failed. Now, this week's had ??? this morning. So, I called dexcom and they said it was a coincidence and there's no such thing as a "bad box." Really? Three in a row? Could it be the summer weather, although I've never had this problem in the summer before. Other thoughts? Thanks!

P.S., I should have added that last week's had ??? 1-2 times each day. :P

I don't think I have ever seen that error. I have noticed that furniture can interfere with reception. If my daughters sits at a metal desk the receiver doesn't get a signal even though it is attached to her belt.

I just joined this site a week ago and have used the Dex cgm since July.  I have had the ??? occur a couple of times.  The first time was a sensor failure and Dexcom replaced it.  Since then, when the ??? happens, I just wait it out and the sensor begins working again.  My sensors have been spot-on when I check with my glucometer.