Ensure you ticked the box "Send a push notification" when you send a manual message.

Ensure you are requesting permission from your users to send push notifications.

Do you get a device token from APNS? If you put a breakpoint into the application:didregisterforremotenotificationswithdevicetoken: delegate call, you should get a token shortly after your app launches.

Have you set the correct Bundle ID in Settings > Channels > Messenger > Install? Make sure it matches the app that you want push notifications sent to.

Is your private key still active? Check your keys to make sure it has not been revoked.

Ensure that notifications are enabled for your app. To check on iOS go to Settings > Notifications > Your App and make sure "Allow Notifications" is turned on.

Is the user reporting issues on the latest version of your app that implements push notifications?

Make sure to tick the "Send a push notification" box when you send a manual message.

Check that the notifications are not disabled for your app on your test device. On the device, go to Settings > Sound & Notification > App notifications. This may differ depending on the Android version.

Did you specify the correct Push Server API key?

Make sure you added your google-services.json file in the correct directory.

There can also be some general Android notification delays due to different manufacturer settings of battery optimization as detailed in this article. There is also https://dontkillmyapp.com/ which details how to modify battery options for different devices. So, ensure that all battery optimization features are turned off when you're testing push notifications.

For Android we currently require Google Play Services to be on the device. There are Android devices without Google Play Services and we do not support push for those.

Is the user reporting issues on the latest version of your app that implements push notifications?