Hi there!
If you are having issues using the double-pinch gesture on your Apple Watch, let's first restart the watch to see if this continues. This refreshes the software and can resolve issues like this. These steps from Restart your Apple Watch show how to do this:
How to restart your Apple Watch if you have watchOS 9
1. If your Apple Watch is charging, take it off the charger.
2. Press and hold the side button until you see the power button
in the upper-right corner of the display.
3. Tap the power button.
4. Drag the Power Off slider.
5. After your watch turns off, press and hold the side button again until you see the Apple logo.
Ensure your Apple Watch is fully updated as well. This installs feature improvements that can also alleviate unexpected behavior. This article walks you through backing up and updating: Update your Apple Watch
If this continues to occur after restarting and updating your Apple Watch, then Apple Support would be the best equipped to look into this further. Contact them here: Apple Support
Cheers!