The following steps may help (check after each):
Remove any protective case that you may be using on your watch, as it may be blocking the microphone port (the lower of the two ports on the opposite side to the buttons):
Clean your watch, paying particular attention to the microphone port, in case it has become blocked by debris.
If you have not yet done so, update your iPhone and Apple Watch to the latest software, starting with your iPhone:
- Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
- Update the software on your Apple Watch - Apple Support
On your iPhone, go to:
- Settings > General > Restrictions - check that the restriction for Siri & Dictation is not enabled.
- Settings > General > Keyboard - check that Enable Dictation is turned on.
- Settings > Siri - disable, pause for a few moments and then re-enable Siri.
Unpair your iPhone from a Bluetooth headset, if one is presently paired to it.
Restart both your iPhone and your watch, turning both off together, then restarting your iPhone first:
- Restart your iPhone, iPad, or iPod touch - Apple Support
- Restart your Apple Watch - Apple Support
Unpair and re-pair your watch:
- The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
- Choose to Restore from Backup when offered the option during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
- Unpair your Apple Watch and iPhone - Apple Support
- Set up your Apple Watch - Apple Support