What are the possible causes of discrepancies in Google Analytics ecommerce tracking?

Question

We have setup Google Analytics Ecommerce Tracking through Google Tag Manager.

Code:

$tracking_script = "
dataLayer.push({
    'transactionId': '$ref',
    'transactionAffiliation': '$cartec_name',
    'transactionTotal': '$amount',
    'transactionTax': '',
    'transactionShipping': '',
    'transactionProducts': [{
        'sku': 'DD-{$amount}-{$currency}',
        'name': '{$cartec_name}_Deposit',
        'category': '',
        'price': '$amount',
        'quantity': 1
    }],
    'event': 'deposit_successful'
});
";
<script type="text/javascript">
    if (typeof(dataLayer) !== 'undefined') {
        <?php echo $tracking_script; ?>
    }
</script>

Tracking works but we have about 200 (30%) transactions missing per day compared to our database records.

What are the possible causes and what are the best methods for debugging this problem?

Answer 1

Your script is running before the GTM's dataLayer object is ready as it's retrieved asynchronously. This means you need to wait the load event is fired/triggered.

Your server side php code is not good as is writing into the dataLayer and definitely is the root issue this code is not awaiting for the GTM's datalayer object. Instead of this just create a variable to be ready for js.

 $tracking_script = "var myObject = {your object code}";

Before the push code you need to put somewhere the php code to create the var

<script type="text/javascript">
  <?php echo $tracking_script; ?>
</script>

Then you can use the load event to properly inject the server side code into the GTM's dataLayer. You can use the next gist:

https://gist.github.com/web20opensource/6c67368a2fdb0e712550#file-load-event-attachevent-addeventlistener-cross-browser

Then just add your lines inside the callback function:

var callBack = function(){
     var myObject = myObject || {};
     dataLayer = dataLayer || [];
     dataLayer.push(myObject);
};

Or use jQuery like

 $( window ).load(function() {
      // Run code
         var myObject = myObject || {};
         dataLayer = dataLayer || [];
         dataLayer.push(myObject);
    });

Answer 2

A 30% discrepancy between Google Analytics and your database is normal as many users use ad blockers which is preventing the script from Google Analytics to load thus to track anything.