Azure says my push notifications have all been sent on to FCM, but FCM hasn't received some of them

Question

Sometimes notifications aren't received, but Azure gives back 'Enqueued' as the result of 'SendTemplateNotificationAsync'. I log everything in a database. I've checked the FCM registration token in the Google Play Console and there is no record of the notifications that didn't get received by the user (and there is a record of the ones that are received), yet Azure said they were 'Enqueued'. I checked the metrics graph in Azure Portal and the stats don't match up at all. Azure thinks they are successful notifications, but they aren't.

Edit for Peter Pan:

1. Sending the notification and checking the outcome from Azure:

   var result = hubClient.SendTemplateNotificationAsync(properties, tags).Result;

   // result is always 'Enqueued', but user doesn't always get the notification

   AddOutcomeToDatabase(result)

2. Checking whether FCM received it:

Google Play Console -> app -> Development tools -> FCM Diagnostics and put in the registration token of the user. I see that the dates he got the notification are there but the dates he didn't aren't there.

3. Checking Azure Portal for the stats:

Azure Portal -> Notification Hubs -> hub -> Metrics and tick GCM errors and select the date range. GCM Errors = 0.