Your device might not vibrate (nor ring) because of misconfiguration of settings. Resetting the app preferences will restore the default settings of all the apps. Follow the steps below in order to learn how to reset the apps' preferences.
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Note: The screenshots are taken using a Sony Xperia M2, running on Android 5.1.1 Lollipop. The instructions mentioned here might slightly vary from one manufacturer to another.
It is very important to update the device's operating system on time as the updates contain fixes to various bugs. If you don't receive a notification about the update, check if one is available manually.
See the don't know how to update my operating system guide if you are not sure how to check for updates manually.
Maybe the vibrator got stuck. A gentle tap on the back of the device should be enough to unstuck it.
If you don’t receive the email in the next 2 minutes please check your junk folder, and add [email protected] to your safe sender list.