Make sure you are syncing the correct app versions on watch and phone. There is a pro and free version of our app and you must sync pro with pro and free with free.
Notifications must be enabled on the device to show notification to start the sync (this is required if the app on the phone is not opened to the main screen, when trying to sync from watch).
Make sure you haven’t done workouts independently for the same day on both the phone and watch. It will ask to overwrite when syncing. In this case the app will use the most recent data for the specific workout day. Also in this case the next workout on both devices may be different. After a successful sync is done, this should be fixed.