FCM notifications

I need skilled developer with FCM notification experience to look for the problem and fix it. We convert everything from GCM to FCM but when I send test notification to single device using device token from FCM console it gives failed! The error showing is Token is not registered in FCM server! FCM server already setup and working perfect! We implemented everything from iOS app side including registering token to FCM server and also our backend side but the problem still with unknown reason! I need someone to fix it now!