title: NotRegistered Response from FCM even when the app is installed on a users mobile device
body: <p>We just migrated from GCM to FCM. For tracking uninstalls we send a silent notification to all our users and if we get a "NotRegistered" response we treat the user as uninstalled. Since the last few days, we started getting "NotRegistered" for some of the users who have not uninstalled the app.</p>
  
  <p>On querying the FCM diagnostics for the same Registration token the messages are being delivered but they are continuously in accepted state.Attached is the screenshot. The error code I received while sending the message is still "NotRegistered" . The user has not uninstalled the app.Attached is the screenshot.</p>
  
  <p>We are getting "Not Registered" since 1st of June while messages are in accepted state till 4th of June. This image shows message Ids of notifications sent till 4th of June:</p>
  
  <p><img src="https://i.stack.imgur.com/sq2y8.png" alt="1"></p>
  
  <p>Would like a clarification on the following:</p>
  
  <ol>
  <li><p>Why did the status of the FCM registration token change to NotRegistered. There was no upgrade since the last month and the user has been using the latest version since a long time?</p></li>
  <li><p>Is checking FCM error code "NotRegistered" a full proof way to track uninstalls? Are there better mechanisms available?</p></li>
  </ol>