FCM - Messages not Delivered

[Pether Pettersson]

We've started to experience a problem where many of our clients phones are not receiving our messages as they should, and during our testing we've noticed that this is a very intermittent problem that we've not found any common denominator for, different devices, different android versions, different and same wi-fi connections, etc, all experience different results.

We've so far tried,

* Priority set to HIGH (HIGH in prod)

* With and without ttl, and set to 0 (prod set to 0)
* With and without collapse_key (not set in prod)
* With and without data, i.e. without either data and notification, and only with data (no data in prod)
* With legacy api, and new api (legacy in prod)
* With and without analytics label set (not set in prod)
* Different firebase projects

* The phones in prod are controlled by a device management system, so battery saving and such settings are the same. For our testing we've tried testing devices and private devices.

This is a problem that a client started experiencing about 2 weeks ago, but have not found any issues reported by google on the status of fcm.

During testing we've seen that the FCM Diagnostics app will say that we are connected, but still not receive any messages, while the phones next to it reports receiving. A few minutes later the missing device will close with error 28 and then connect and retrieve the missing messages.

We've opened a ticket to support, but will open a thread here as well if anyone else has any suggestions.