As long as you have a backup on the phone, it backs up with the phone's backup to the cloud. If, however you want (as I do) the separate backup to the cloud, try going into the "restore" section and deleting any current watch backup that's already in the cloud (not the one on your phone - when you click restore, it should show 2 watch backups: phone and cloud). Reboot your watch and then try forcing the cloud backup again or wait until regular backup time.
You can also use Smart Switch to back up the watch to a computer or the cloud via computer connection.
