There might be a situation when some of the users do not receive push notifications that you have been sending to them. There is a set of possible reasons for this. You must figure this out.
Here is the common problem comes with the push notification.
1. Failure in connecting to GSM: This problem arises when a user not using an internet for a long period of time at a particular mobile phone or system.
2. Firewall Issues: companies use a firewall for their protection and this is the also reason to block unknown notification from outside.
3. Blocked at the operating system level
4. Battery Saver Mode