I have been having an occasional issue with my Garmin 520 not syncing rides with Garmin Connect for awhile now. It only happened once in a while, but recently its happening most of the time. I was recently away on vacation, and could not get rides uploaded. We had 3 laptops between the people there but all of them were "work" laptops which prohibit connecting USB devices so I was unable to upload rides for several days until I got home.
To remedy this, I have tried nearly everything. Phone software up to date. Garmin Connect up to date. Garmin Edge 520 software up to date. Restart the iPhone 7. Restart the Garmin 520. Uninstall and reinstall Garmin Connect. Delete rides around the ride that won't upload. Create and save some new fake rides (those sync immediately without even asking). I even tried syncing to Garmin Connect on a different device. If I ever get to the point where I have to manually sync (select Upload Ride on the 520), the ride will never upload. No matter what I do, it will start to upload, the green dot appears on the 520, but then it makes the circle rapidly, and that's when I know I am doomed -> Upload Failed after about 30 seconds.
I am about to try some drastic measures. I am going to remove all the rides and courses, save off my profile info and do a device reset. Seems crazy but I don't know what else to do.
Gary