We noticed that on some iPhones, even with the latest version of iOS and with the bluetooth connection turned on, the watch won't synchronize.
Some articles (link) relate similar issues with other bluetooth devices.
Usually, this step-by-step process solves the problem:
1) turn the bluetooth connection OFF,
2) then turn it back ON.
3) delete the watch
4) pair the watch again
Then, synchronization should work. If not, reboot your phone by long-pressing the on/off button.
Please try this procedure and if it is still not working, send us a ticket using the link at the top right hand corner of this page.