To do so: Go to Settings > Applications > Instagram > Notifications and activate “Allow Notifications.” If the above does not solve your Instagram notification not working problem, the Clean Master or similar security and performance applications that you have installed on your phone may be preventing you from receiving notifications. For example, the Clean Master application has notification blocking, which you can disable or delete.

https://www.instafollowers.co/blog/how-to-fix-instagram-notification-not-working