If your watch stops counting steps, it's a good idea to do a general check up to make sure everything is set up in the right way.

First of all, make sure to update to the latest version of StepsApp available on the store: https://apps.apple.com/at/app/id1037595083

Then follow these steps to update your device's OS:

• Open Settings app
• Go to “General”
• Check Software Update

Afterwards restart the Apple Watch: https://support.apple.com/en-us/HT204510

And also restart your iPhone: https://support.apple.com/en-gb/HT201559
 
If the aforementioned actions didn't help, please unpair your watch and pair again: https://support.apple.com/en-us/HT204568

At times, you might need to uninstall and re-install the app from the Apple Watch.