iOS Push Notifications with Firebase Topic Messages are not making it to the device

I'm attempting to send an iOS push notification to a topic in Firebase using node.js. I followed this tutorial but can't figure out why the notification isn't making it to the devices subscribed to the topic. I am able to send topic messages from the console and listenForNotificationRequests() is removing the "notificationRequest" children successfully.

Here is the structure of the notification request in Firebase: example notification request

Here is the node.js code with keys/urls removed:

var firebase = require('firebase-admin');
var request = require('request');

var API_KEY = "APIKEYREMOVED"; // Your Firebase Cloud Messaging Server API key

// Fetch the service account key JSON file contents
var serviceAccount = require("./pathToJSON");

// Initialize the app with a service account, granting admin privileges
firebase.initializeApp({
  credential: firebase.credential.cert(serviceAccount),
  databaseURL: "URL_REMOVED"
});
ref = firebase.database().ref();

function listenForNotificationRequests() {
  var requests = ref.child("notificationRequests");
  requests.on("child_added", function(requestSnapshot) {
    var request = requestSnapshot.val();
    sendNotificationToUser(
      request.username,
      request.message,
      function() {
        requestSnapshot.ref.remove();
      }
    );
  }, function(error) {
    console.error(error);
  });
};

function sendNotificationToUser(username, message, onSuccess) {
  request({
    url: "https://fcm.googleapis.com/fcm/send",
    method: "POST",
    headers: {
      "Content-Type" : "application/json",
      "Authorization": "key="+API_KEY
    },
    body: JSON.stringify({
      to : "/topics/user_"+username,
      priority : "high",
      notification: {
        title: message
      }
    })
  }, function(error, response, body) {
    if (error) { console.error(error); }
    else if (response.statusCode >= 400) { 
      console.error('HTTP Error: '+response.statusCode+' - '+response.statusMessage); 
    }
    else {
      onSuccess();
    }
  });
}

// start listening
listenForNotificationRequests();

Any help/advice is greatly appreciated!

via Spencj