It could be because of the settings that are done on your device. You can change the below settings of your device in order to receive notifications.

Method 1

Please go the phone's settings, look for 'Battery', click on 'High Background Power Consumption' and then enable 'PetBacker' app to receive the notifications.

OR

Method 2

Step 1:

Enable "Auto-Start" for PetBacker

Step 2:

Go to i manager --> App Manager --> Autostart Manager --> Enable PetBacker App

or

Settings --> More Settings --> Applications --> Autostart --> Enable for "PetBacker"

Step 3:

Click on App Manager

Step 4:

Click on Auto Start Manager

Step 5:

Enable PetBacker

Step 6:

After following the above steps come back to :-

App Manager --> Tap on Uninstallation / Installed App --> PetBacker --> Enable Show Notification

or

Settings --> More Settings --> Application --> PetBacker --> Enable Notification