Google Firebase - Cloud Messaging - some messages fail to show the pop up notification

I have set up google javaScript firebase cloud messaging client. I set up both onMessage (called when app has focus) and setBackgroundMessageHandler (Handle messages when web app is in the background). The problem is when i push a message to FCM server to push to my js client, the pop up only shown for some push requests only, some won't show the popup even though onMessage get called every time pushed a message.

http://ift.tt/1TmHkUo
Content-Type: application/json
Authorization: key=AIzaSyC...akjgSX0e4

{ "notification": {
    "title": "Background Message Title",
    "body": "Background message body",
    "click_action" : "https://dummypage.com"
  },

  "to" : "eEz-Q2sG8nQ:APA91bHJQRT0JJ..."

}