What do you mean by not syncing? Not getting the health data from watch to the phone? Not getting notifications? Not backing up to the phone? Not connecting to the phone at all?
Did you reboot both devices? You might have to try unpairing it from your phone and unlinking it from the wear app and reconnecting it.
Well, it appears to be transferring data, although the older it gets, the more prone it is to losing portions of my sleep data. But now the app just tells me it isn't syncing. When I open the app it tries, then tells me it hasn't synced since the 24th. So I really am not clear why it says that. I have rebooted both the phone and the watch, but not at the same time.
Have you tried deleting the cache partition on the watch? Hold both buttons until the watch restarts and let go of them when you see the Samsung logo on reboot. Use the back key to scroll up/down till the option to delete cache partition is highlighted, press the home key to select, repeat to highlight and select "yes", then select reboot when the partition has been deleted. See if that solves the issue. I don't think it's the age of the watch; I have the watch 4, which is older, and my watch is syncing perfectly.
