Push notifications not being sent

Hello, I have been working on a chat application in the past month, and I finished everything I planned on doing, and everything was working fine (a week ago). But now when I log in the app and attempt to send a message, the messages are just not being sent to other devices.

The way my app work is I used a custom businesses logic, and so to send a message you must make an API call through the server which publishes the message. Now in the past few days I have been trying to send a message and it is just not being sent to the devices. I can see the messages being stacked up through the Message Console, but my custom push receiver is not being invoked at all.

Here is a diagram that shows the expected result and the current result:

Expected:
Client A -> API Call with arguments to the cloud code -> Cloud Function process the information and publish the message -> Push receiver is invoked in Client B.

Current State:
Client A -> API Call with arguments to the cloud code -> Cloud Function process the information and publish the message.
Push receiver of client B is not invoked.

This is the only issue I am having with the server at the moment, everything else is working fine, I can load all the files and even upload files, however in a chat app, the whole core is depended on the push notification system, to which I need a fix ASAP.

Update:
Solved this issue by replacing the GCM API key:
I had an Android API Key in the console (It was something that always used to work), And so I replaced it with a Server Key, and now everything is functional again.

I don’t know why is occurred and why is this the fix, perhaps you updated your services and you changed something in the config for GCM and push notifications?

Actually, the documentation mentions that you need a Server Key in order to receive push notifications: https://backendless.com/documentation/messaging/ios/messaging_push_notification_setup_androi.htm