Hello shivamjaggi,
Do you know if your iPhone is set up to vibrate on ring and silent? Per How to put your iPhone on vibrate, ring, or silent mode - Apple Support
Choose when your device vibrates
1. On iPhone 7 and later, go to Settings > Sounds & Haptics. On earlier iPhone models, go to Settings > Sounds.
2. You can choose if you want your iPhone to vibrate when set to Ring or Silent mode. If you turn off both settings, your iPhone won't vibrate.
If so and issues are ongoing, then you'll want to contact Apple Support so that they can look into this directly with you.
Thanks for using Apple Support Communities!