You may have had issues with Google Adwords duplicating your transactions and thus not reporting conversions correctly. If this is the case, it may help to prevent the user from reloading the Thank You page. Below is a script that you can add to your Footer block in the Theme options.
<script>
if(location.pathname == "/checkout/thank-you") {
(function () {
$.ajax({
method: 'get',
url: '/This-will-kill-the-session'
})
})();
}
</script>
Google Adwords Duplicate Conversions should hopefully be solved by implementing this functionality.