I found a fix! I had to reboot the watch. This isn't obvious since the watch has never run out of battery for me. I just held down the light button for a few seconds and it gave me an option to shut off. When it came back - sync worked.
For clarity - my bluetooth was connected prior, notifications worked but it had stopped syncing data for 2 weeks. I have a 735XT and running latest IOS on an iphone 7.