phone has stopped vibrating when put in silent mode

The Ring/Silent switch is on the left-hand side of your iPhone.

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Change ringtones, sounds and vibrations

1. On iPhone 7 and later, go to Settings > Sounds & Haptics. On earlier iPhone models, go to Settings > Sounds.
2. Choose what sounds you want to adjust, such as Ringtone or New Mail.
3. Tap the alert tone you want.

How to put your iPhone on vibrate, ring or silent mode

Check your settings in the notification settings disable vibrations