Are you being taken to the shipping failure page?
To get a better idea of what is gong on you will need to ad in the error message bindings on your shipping failure page. In the bindings panel you will need to expand the USPS and find the error messages and codes then drag them onto your page. The next time you perform a test it should let you know what the problem is. Please post back to us with the error you are getting if you can't resolve the problem.