Make Sure Ringtone Vibration Is Turned On

1. Open up the Settings app.
2. Scroll down and tap Sounds & Haptics.
3. Under the Sounds And Vibration Patterns submenu, tap Ringtone.
4. Tap Vibration.
5. Make sure the vibration pattern is set to anything but None.
6. The vibration pattern you have selected will have a small check mark next to it.

Why this works:

Even if Vibrate on Ring and Vibrate on Silent are turned on, your iPhone won’t vibrate if your ringtone’s vibration pattern is turned off.