There's many reasons why conversion data goes missing, most of which is out of our control. Below are good resources to reference to help understand what is going on.
Crazy Egg - http://blog.crazyegg.com/2013/01/31/why-is-google-analytics-inaccurate/
Google Analytics - https://support.google.com/adwords/answer/2404039
Since Google Analytics captures data on the client side, it's susceptible to the following behavior.
- Use of a script or ad blocker.
- The buyer navigates away from the order confirmation before Google Analytics can record data.
- Your website is not SSL secure. This causes referring data to disappear.