I have the same issue now for the 2nd time.
I can sync with GE with my iMac but with the GC on the iPhone the sync is "frozen". I had to do a reset with complete deletion of the settings and the data. After setup and reconnection with GE and GC it works again.
BTW. The steps, heart rate, ... is synced but not the activities.
I use Android phone.
But finally fixed coonectivity problem.
After trying everything (cleared everything and lost ~ week of my activity data :/) to morrow i deleted every bluetooth paired device from phone bt seetings. And it worked! I was able to connect watch again!