We have found that currently the Forerunner 15 will only sync step data from the current day. If you sync your data less frequently what will likely happen is the step data for any previous day besides the day that you sync will not upload to Garmin Connect. ​
This issue has been reported and is being addressed with high priority, until this time the work around at is to sync your device daily.
As you can see from the attachment, my Forerunner 15 will not sync with GC. I have been away from home for three weeks and upon my return, I found that there is a new version of GE (currently I'm running 4.0.5.0). I updated and tried to sync but was unsuccessful. In debugging the issue, I have completely uninstalled GE and hard reset my device and the sync problem continues. I'm currently located in Kazakhstan and have tried to sync with and without VPN (New York server) enabled, both fail.
Prior to vacation, I was unable to sync data properly and submitted a forum thread [1] regarding that issue. One of the comments was that Garmin confirmed it to be a Garmin issue, not client. Does anyone have a working configuration with a Forerunner and GC via GE? I sync once per day and can manually upload activities, but I'm not sure if I can do the same with the wellness data. Any help would be appreciated. Thanks!
[1]: https://forums.garmin.com/showthread.php?201547-Data-not-syncing-properly