Bizarre problem I notice recently. I went for a run earlier today, and the training saved just fine on the watch (I can go back to look at the data for it on the watch directly), but it refuses to sync with the Activity app on my phone. Strangely, the distances corresponding to the run appear just fine during the Walking + Running Activity in the Health app.
Even stranger, I’ve noticed that the Activity app on my phone is actually missing all my, well, activity for the last eight days, so the rings are completely dark (I only noticed today because I moved this week and did not have not time to fit a run in). Again, health shows all the raw data just fine.
I’ve tried pretty much everything suggested for syncing issues, including resetting both devices, backing up my phone to iCloud, checking my workout activity privacy settings, closing all related apps, and then resetting the devices, changing my weekly activity goal on clock, and make sure iOS is up to date. This also seems to be the only problem with connecting the phone; I’ve been using the clock all week to check my music / podcast apps, check my email, and view calendar events.