1. It may be that the iPhone with the watch is low in power or the latest version of iOS system is not installed.

2. It may be that the connection with the iPhone is unstable, or there is a problem with the Bluetooth connection with the device.

3. It may be that the "wrist detection" function is turned on, and the screen will automatically lock when it is detected that the user is wearing a watch.

4. It may be that the "hand washing" mode is turned on, and it is detected that the user will automatically lock the screen when washing hands.

5. You can turn on the "Sleep" mode, which will automatically lock the screen when the user is detected to be asleep.

6. You can turn on the "Fitness Record" function, which will automatically lock the screen when it is detected that the user is doing fitness activities.

7. It may be that the watch screen is damaged or there are other hardware failures that make it impossible to automatically lock the screen.

If the above methods fail to solve the problem, it is recommended that you contact Apple customer service or go to an authorized repair point of Apple for overhaul and maintenance.