sync problem with Garmin
i had same sync problem : activity from Garmin 830 was not syncing with Strava. Here is solution: you need to choose activity that has preset in Garmin: Biking. And it will sync with Strava. DO not create your own activity , like walking - it will not be synced. ( That what happened to me) Instead if you are walking - choose Biking in Garmin and after the end of the activity go to Synced/uploaded activity in strava and change type of activity. It worked for me.
P.S. All that connect / disconnect - use Safari or Chrome - all non-sence. Problem is - type of activity. Strava does not recognize custom activity from Garmin.
Good luck !