For me it was an issue with deviceToken. Please check if the receiver and sender device token is properly updated in your database or wherever you are accessing it to send notifications.
For instance, use the following to update the device token on app launch. Therefore it will be always updated properly.
// Device token for push notifications
FirebaseInstanceId.getInstance().getInstanceId().addOnSuccessListener(
new OnSuccessListener<InstanceIdResult>() {
@Override
public void onSuccess(InstanceIdResult instanceIdResult) {
deviceToken = instanceIdResult.getToken();
// Insert device token into Firebase database
fbDbRefRoot.child("user_detail_profile").child(currentUserId).child("device_token")).setValue(deviceToken)
.addOnSuccessListener(
new OnSuccessListener<Void>() {
@Override
public void onSuccess(Void aVoid) {
}
});
}
});