The Activity app on my iPhone always shows the "setup" screen: "Set up Activity on your Apple Watch". I have already set up Activity on my Apple Watch; it works fine and shows the rings filling up each day. But I cannot access historical Activity data which should be displayed in the iPhone app.
I have tried restoring the watch from a backup and setting up the watch as new. Neither has worked.
A little background: I bought my Apple Watch along with my iPhone 6s Plus. That initial pairing worked without any bugs in Activity on phone or watch. However, two weeks after purchase, my iPhone experienced some kind of catastrophic failure. I got it replaced at the Apple Store. Since pairing my watch with the replacement phone, the iPhone Activity app has never recognized that I have already set up the Watch Activity app.