This happened to me, after upgrading my Apple Watch from series 3 to series 9.
This is everything I did to solve my problem. I'll list the steps in a nutshell, then I'll list the ones that I think helped solve the problem in detail.
- I erased my apple watch
- setup my apple watch as a new (not from backup)
- connected to my Apple Fitness+ on my Apple TV
- I restored my watch from backup
- Reconnected my watch to Fitness+
- closed the Fitness+ app
- connected to the Fitness+ app again to to make sure it still worked.
I think the steps that were helpful were steps 4 through 7 which I'll detail here:
Restore from backup
- on the watch, go to Settings > General > Reset > Erase All Content and Settings.
- Enter your watch Passcode to confirm.
- Bring your iPhone near the watch and follow the steps on your iPhone screen to Restore from backup.
Connect the watch to the Fitness+ app
- Open the Fitness+ app on your AppleTV
- when it asks, "Who's working out?" choose "Other"
- Open the Workout app on your Watch. It's the silhouette of a person on a lime-green background.
- tap the Connect button on your watch.
- wait about 20 seconds or so to connect.
If unsuccessful, try erasing the watch without restoring from backup, then connecting to the Fitness+ app like I did, then restore from backup and reconnect.
If successful, close the Fitness+ app, open it again and connect but selecting your watch instead of selecting "Other". It should connect in under 10 seconds now, like normal.