My workout on Apple Watch isn't showing up in my History

Strong for Apple Watch syncs your workouts automatically each time you open the app on your Apple Watch or complete a workout.

Strong uses the internet to sync workouts, rather than communicating directly between iPhone and Apple Watch. This is why workouts may appear in the Health and Activity apps before they show up in Strong.

To force Strong to attempt to sync your workouts immediately, you can do a “pull-to-refresh” gesture on the History tab, or use Settings > Apple Watch > Download Workouts. For best results, have Strong for Apple Watch open to your list of routines with the screen on.

In previous versions of watchOS, we also recommended launching Apple’s Remote Camera app on your Apple Watch if you thought that you might be having connection issues. This doesn't seem to have any effect in watchOS 4.