I have the Watch 4 Classic and use it with my S21 and mostly things work fine. But since setting up, and calibrating the blood pressure measuring it synced the measurements taken fine the first day I used it, but has refused to sync them every day since. What's up with that?
I did not change any settings and all available updates are installed on both devices.
Go to Samsung Health Monitor app, press on the three lines in the top left corner, select Data and then press on the Samsung Health option. In the Data Permission section, set all permissions to on. This will solve the issue.