My Vivosmart 4 has not saved my data since 7-7-2020, it is not recording my sleep or pulse oxygenation,, and will not sync to the Weight watchers app. WiFi and Bluetooth is on. I wear my device 24/7 unless it is charging. Very frustrated and disappointe

With a multitude of issues like this, suggest contacting Garmin Support. Could be a faulty device and covered under warranty.

Other than that, are you able to see any data in Garmin Connect Web? Also, some users are having success with the Sync / Connection Troubleshooting Guide

I find that that my Vivosmart regularly stops syncing history after a recorded activity and will only resume by recharging the device (which reboots it). Garmin Support keep saying I should tweak anything from Bluetooth to battery optimization on my phone, which I've already done, to zero effect. The ONLY thing which works is to plug the bracelet to a charger - regardless of its charge level. Sometimes I go days without a problem, sometimes it's the first activity since the last restart which screws it up again.

Garmin Connect / Settings / Device Sync Audit will will show a "SYNC RESULT - FAILED (Failed to read file from device)" when this happens.

I encounter the issue with not syncing after an activity once in a while. You don't have to put the device into the charger. It usually helps to dis- and re-enable Bluetooth on the VS4. After that, sync is back to normal.

Doesn't help for me. Nor does force quitting Garmin Connect. What I haven't tried is rebooting my phone, because first of all that'd still be a Garmin fault, and second, I don't think it'd actually help given the other symptoms.