Tried to sync today's run from my 910xt and it tried to sync loads of stuff that had already uploaded but completely failed to do todays. It's still on the watch but it can't see it. You used to be able to manually download but can't see any facility to do that now.
Please make sure Express is up to date, delete activities from the watch that are already uploaded to Connect then use the Transfer All option to resend the Activity in question.
How do I update Garmin Express?
How can I resend my history from my wireless watch to my computer?
Please let us know if you continue to have troubles. Curt