Hi
The following steps may help (check after each):
If you have not yet done so, update both your iPhone and your Apple Watch to the latest software, starting with your iPhone (be sure to make a backup first):
- 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 > Siri & Search - check the all desired settings are enabled.
- Go to: Settings > General > Restrictions - check that the restriction for Siri & Dictation is not enabled.
- Go to: Settings > General > Keyboard - check that Enable Dictation is turned on.
Check that your Apple Watch is connected as required:
- How to use your Apple Watch without your iPhone nearby - Apple Support
- About Bluetooth and Wi-Fi on Apple Watch - Apple Support
Remove any protective case or film that you may be using on your watch if it is blocking the microphone port (the lower of the two ports on the opposite side to the buttons):
Clean your Apple Watch, paying particular attention to the microphone port, in case it has become blocked by debris.
- Be sure to follow Apple's cleaning instructions.
- Cleaning your Apple Watch - Apple Support
- Important handling information
Restart your Apple Watch and iPhone, turning both devices off together, then restarting your iPhone first:
If the issue persists, next try unpairing and re-pairing your watch:
- The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
- Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions such as Apple Pay cards and passcode).
- If the issue persists, next try unpairing and setting your Apple Watch up as a new watch.
- Unpair your Apple Watch and iPhone - Apple Support
- Set up your Apple Watch - Apple Support
If the issue then still persists, I suggest that you contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance: