This may have happened because you need to update the software on your devices. First, please be sure you have updated your phone to 12.0 or later. Go to phone ‘Settings’, scroll down and tap ‘General’, then tap ‘Software Update’ and proceed with the Apple software update until finished.
Then, be sure your Apple Watch software is 5.0 or later. Open the ‘Apple Watch’ app on your phone and tap the ‘My Watch’ tab, scroll down and tap ‘General’, then go to ‘Software Update’ and proceed with the Apple Watch updates.
This should also help Seven run smoothly and sync between your devices.
Also, please note that watchOS 5 requires iPhone 5s or later with iOS 12 or later, and Apple Watch Series 1 or later. While, watchOS 5 isn't compatible with Apple Watch (first generation).