The same thing happened to my wife as well, with three different Inspire 3 devices, and the Customer Service team was unable to get any of those devices to work so she finally returned the last one and then gave up altogether. My Inspire 3 works fine, however.
The first thing that the CS team had her try was to toggle the Heart Rate switch in the Settings menu on the device, because the devices always indicated that detailed sleep results weren't returned because they couldn't get consistent heart rate data overnight. That didn't work, and the heart rate chart showed no breaks at all in the curve overnight, so that was clearly not the problem.
I didn't know about the Fitbit online dashboard board back then, so anybody that is experiencing missing data issues should definitely go there to see if the data is missing there also.
If the data is on the dashboard but not on the app, then it is a sync issue. If the data is not on the dashboard, then the problem must be related to the device and/or app.
In my wife's case, the devices that didn't work never worked, so she had to return them for a replacement. Clearing app cache and reinstalling the app did nothing to resolve the issue.
