I noticed a lot of people who have been having a problem where google checkout was processing sales, but they weren't showing up in the Zen Cart admin.
On the google code page for google checkout:
http://code.google.com/p/google-checkout-zencart/

There is something called "responsehandler_test_v1.2.php"
if you download this script and place it on your site, you can get a lot of useful information about what's taking place when a transaction occurs.

This helped me solve my own problem (in my case, a certain table had not been added to the database when I first installed google checkout). It could be useful to your problem as well.