If only some record don't make it to the Google Sheet, consider the possibility that the user closes the course before all data is sent. The internet is not instant, and the data is sent one field at a time. So, for example, if you're sending a bunch of data on the last slide before someone exits a course, it's quite possible that they exit before everything is sent.

Let me know if this helps or if we need to do more troubleshooting.