Hook is not executed for all new signups

I have a hook setup and it works fine in testing mode and also works for some signups but not for all. Many of my new users is not send to my Mailchimp API where I used to add them to a newsletter.
It is very difficult to debug as I cannot see my logs more than 2 days back.
My last user fired the hook on the 1st of September, but there are others after that date. I suspect that the users with social signups are the ones not triggering the hook, but just a guess.

var request = require('request');

module.exports = function (user, context, cb) {
  // Perform any asynchronous actions, e.g. send notification to Slack.
  var apiKey = "<MY_API_KEY>";
  var urlMailchimp = "https://us11.api.mailchimp.com/3.0/lists/<LIST_ID>/members";
  var userMailchimp = "<MYEMAIL>";
    method: 'POST',
    url: urlMailchimp,
    headers: {
      'Content-Type': 'application/json',
      'authorization': "MY_API_KEY"
    body: `{
            "email_address": "${user.email}",
            "status": "subscribed",
            "tags": ["TowerUp user"],
            "merge_fields": {
              "MMERGE3": "${user.family_name}",
              "MMERGE1": "${user.given_name}"
  }, function(error, response, body) {
    cb(error, response)

Could you please check it out?

It would be more helpful if you try to reproduce the issue with social signup so that we can drill down the issue.