Nokia Lumia 820 - My phone does not vibrate properly

  1. Connect a charger to your phone, this is to bypass possible power savings mode.
  2. Select a different ringtone to recognise how the vibrator follows the sound. 
  3. Check if the issue is with a specific number on incoming calls.
  4. Restart your phone. You should feel a vibration while the phone switches on.

Note that if you switch vibrating alert on, your phone will vibrate even if it's ringing option is set to Silent.