Unable to Registering the Device-Push notification

Hi all, I am working on Push notifications and most of the devices was registered properly but some of the devices was not registered .i have added break point and checked the where  the devices object was creating  but it was not hitting the microflow. unable to find the root cause of why in only few mobiles it was not registering the device but the application was installed sucesfully. Any help appreciable, Thank you
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