We've added this code exactly as provide on Marketing Cloud to Google Tag Manager and set it to run on every page view. However, only about 10% of visits are being tracked. I've replaced our OrgId with XXXX in the example below...
<script type="text/javascript" src="http://XXXX.collect.igodigital.com/collect.js"></script>
<script type="text/javascript">
_etmc.push(["setOrgId", "XXXX"]);
_etmc.push(["trackPageView"]);
</script>