It's really frustratinh

Open S Health, click 'Backup and Restore', wait a minute (S Health will restart during the process), and Boooiiingg: say Hello to all the workouts you did in Workout Trainer in the past, together with the time the Workout took and the calories you burnt. Unfortunately, as long as Samsung is taking to fix this with an update, you will have to do the procedure again when you finished new workouts: it only syncs past ones.

Edit: this worked all the time, but when I tried it myself recently, it doesn't anymore! Screw Samsung!

Yeah!  It just stopped working for me too!  really sucks!