Apple Watch SE (2nd generation) will not sync activity or data

When the Apple Watch SE (2nd generation) stops syncing with the iPhone, the first thing to check is the connection itself, and Apple's page on a watch that is not connected or paired lists the checks. On the iPhone, make sure Airplane Mode is off and Bluetooth and Wi-Fi are on; on the watch, check that Airplane Mode is off and that the red iPhone icon, which means no connection, is not showing at the top of the watch face. Bring the two close together. Restart both devices, since a stale Bluetooth link is the commonest cause. Then check Watch app, My Watch, that the watch is listed and connected. If activity, messages or heart-rate data are missing, give it a few minutes after reconnecting.

If it still will not sync, unpair and pair again, which Apple's pages describe, with a backup first. Syncing of Health and Activity data also depends on settings in the Health app, and on iCloud Health being on if you want the data on other devices. If messages or notifications reach the iPhone but not the watch, check that the watch is on your wrist and unlocked, since the iPhone sends alerts to the watch only when it is worn and unlocked and the iPhone screen is off. If the problem is limited to one app, open that app's page in the Watch app, My Watch, and check that it is set to show on the watch and that its notifications are allowed.