My son, 11, has been on the Dexcom for 2 1/2 years. In recent months, we have been getting ??? during the initial two hour calibration period immediately following insertion, and then usually again at least once during the week. The ??? are always appearing at night, as we insert in the evenings usually between 8 & 9 PM and within an hour to an hour and a half, while my son is drifting off to sleep, the ??? appear. Later in the week if we get the ???, which we usually do, it happens while he is sleeping. This is a new phenomenon. In the previous two years, we’ve seen the ??? probably fewer than 10 times. It is now happening with every sensor since early December. We called Dexcom and they replaced a box of four sensors, thinking maybe there was an issue with manufacturing of the lot, or perhaps they were left in the cold for too long during shipment. Unfortunately, this is not the cause, as it has happened now with sensors from three different lots. When the sensors are sending readings, they are fairly accurate by day 3 or so. Anyone else ever experienced this? Could there be something going on while my son is sleeping? Could other electronics in the area (laptop, cell phone for Share) be interfering? Could the prob be with the transmitter or receiver? Could it be that my son’s body, as he enters puberty, is causing the issue? I’ve asked Dexcom all these questions and I can’t get an answer or even an indication of what the possibilities are. Any help from others who experienced something similar would be appreciated. THANK YOU!
I get the dreaded “???” when I have a compression problem caused by sleeping on top of my sensor. Usually the readings go low with clearly false readings and then it drops out with a “???”
Compression could be an issue as @Brian_BSC says. Inadequate hydration could also be a cause. My son, Caleb has had ??? at odd times. It doesn’t last very long. I encourage him to drink - not that he always responds to that encouragement.
Compression could most definitely be the cause. Have you tried maybe switching his spot? I know diabetics tend to favor areas but sometimes switching up could help. I know I have to force my boys to inject, test other areas than their “go to” spots. Dexcom customer support has normally always been very helpful when I’ve had to call. However I know that doesn’t mean the problem is always solved. Last week we had to reset my sons, by pushing a paper clip into that small hole on the back. But be warned, it starts it over so I would only do it when you are doing a sensor change. You just hold the paperclip in until the receiver vibrates.
Is his receiver close to the 6 month expiration date? Maybe that could be an issue? I know when we calibrate against their meters verses the Dexcom they vary BG ranges by about 10 or so. It does seem to take a few days for that gap to close. Hoping you find answers! Much luck to you.
I´ve had that same ??? on my screen, usually resolves after drinking water as Lorraine mentioned. I used to change sensors at night but found very unpleasant those ??? in the middle of the night.
How much water are you talking about? Does this mean drinking a while before every insertion or constantly like we are all supposed to do?
Yes, like we are all supposed to. But even so, it sometimes appears (???) and if I take a couple of glasses it works fine.
The “LOW’s” in the middle of the night are not fun. The sprints from our bedroom to my son’s bedroom due to compression of the sensor while sleeping are not a good feeling; the first couple of times it happened we would treat and then check. 
Yes! That is exactly how it happens when we get the ??? midweek. My son’s BG nosedives over a 10-15 minute period and the ???s follow. One time I was able to calibrate before the ??? appeared (but after the CGM showed signs of a false nosedive) and we didnt’ get the ???. But I’m not often awake at 2:30 am to catch the problem before it occurs. Good to know this is likely a compression issue. I’ve never heard of this before. Thanks!
We get exactly the same issue when the location of my son’s Dexcom makes him roll onto it at night.
For instance, a few weeks ago we placed a sensor a little too far to the right on his right thigh. A a result, when he rolled over to his right side at night, he would put all his weight on it, and get into a compression low about 5 times per week, after which we would lose signal for 30 to 90 minustes.
In our experience, the only way to deal with it is to pick a sensor location where there is little likelihood of compression into the sensor. For him, the abdomen and the front part of the thigh work for the that - we think the back of the arm would also work. The side of the arm and the side of the thigh don’t work for him.