While I can't offer a fix, I can say I have periodically experienced this on my own Charge 2. I have tried adjusting the position on my arm (relative to my wrist) as well as adjusting the tightness of the wristband (I use a stainless magnetic band). There doesn't appear to be any logic to the failure mode. All of a sudden it works, and then maybe not! Since I also have random loss of sleep stage reporting, I assume there are just some conditions that randomly occur that cause this.
