Dexcom G5 issues

You have to calibrate once every 12 hours, so twice a day. I'd avoid callibrating too often, don't try to callibrate more than 4 times a day, you'll just confuse it.

What's your receiver, is it your phone or a dexcom receiver? I've used both with a G5 and find the dexcom receiver is more reliable/consistent. If it's your phone you might want to have a look at what other Bluetooth connections it's trying to make, and minimise them, and also make sure you've not got anything in between interfering with reception, like don't stick your phone on top of speakers on bedside table.

If it's on the first day it might occasionally be less accurate as the sensor settles in. You could have had a compression low - if you lie on it, compressing it, it makes the sensor read lower than you are.

The biggest source of my ??? and false low readings are caused by me sleeping on my sensor. They're known as compression lows and they can cause your dexcom to yield incorrect readings.

I turn in my sleep often and as a result place my sensor on the side of my abdomen up near my ribcage. That allows me to sleep on my stomach, back and other side without compressing the sensor.

That would be my guess as to why you're having trouble with your system.