This is a belated attempt to try and figure out why my Instinct Solar will not sync data with Garmin Connect but will sync tracks and waypoints with Garmin Explore.
My phone sees the watch through both apps, so it doesn't appear to be an overall connectivity issue. However, in Connect, a sync attempt completes immediately and logs indicate there is no new data available on the watch. If I look at the watch's flash storage from a PC, I can see files are being updated. I'm able to access my tracks in Garmin Explore, which syncs fine, but don't have access to longitudinal heart or sleep data, for instance.
This started August 31 and two things happened about that time. First, my daughter got a Forerunner 35, which I temporarily paired to my account, so I could get a firmware update. Secondly, and the more likely cause, the watch shutdown when the battery was low. It seems like after that event, the syncing with Connect ceased.