I'm missing Workouts performed on my Apple Watch
Normally, completed workouts will show up in History on your iPhone within 5 minutes. Strong will try to sync workouts automatically when you launch it on your iPhone or if you "Pull to Refresh" your workouts in the History tab. Workouts will generally show up in Activity on your iPhone before they're loaded into Strong on your iPhone.
Sometimes, you may need to go to Settings > Apple Watch > Download Workouts in order to prompt your Apple Watch to upload its workouts. It can be helpful to have Strong open on your Apple Watch on the list of routines, and with the screen on before you tap this option.
Please try restarting both devices if this doesn't work. The easiest way to do this is to hold down both the Digital Crown and Side Button on your Apple Watch until the Apple logo shows up. On iPhone, hold down the Power Button and Volume Down Button (or Home Button for older models) instead.