Google forms allow fields to be designated as required. When running the form using the standard Google URL, the form will be validated and presented back to the user if any of the required fields are not entered. There is an issue (aka bug) with required fields in the current version of the plugin.
If a user submits a form that is embedded in WordPress using the plugin, because there are missing fields the form processing does not complete. There is no indication for the user that something is wrong, the form is simply presented again without any of the fields having data in them.
To resolve this I am looking at doing form validation on the client side using a jQuery plugin. Hopefully I will have this resolved fairly quickly as I need this to work correctly for my own project. My testing didn’t take into account a user not completely the form correctly. Oops. This bug affects all versions of WordPress Google Form up to and including v0.9.