Did you try the general troubleshooting mentioned here?
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If Siri doesn't respond when you say "Hey Siri," here are a few things to check.
Check if "Hey Siri" is on
- Go to Settings.
- Tap Siri & Search.
- From here, make sure the following settings are turned on:
- Listen for "Hey Siri"
- Allow Siri When Locked
Set up "Hey Siri" to help Siri recognize your voice
- Go to Settings.
- Tap Siri & Search.
- Turn off Listen for "Hey Siri," then turn it back on.
- When the Set Up "Hey Siri" screen appears, tap Continue.
- Say each command that you see on your screen.
- Tap Done.
If Siri still isn't responding
By default, Siri won't respond when your device is face down or covered unless you go to Settings > Accessibility > Siri and turn on Always Listen for "Hey Siri."
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copied from: If "Hey Siri" isn't working on your iPhone or iPad - Apple Support