When no quotes are diplayed by USPS, tuen on the debug in the VirtueMart configuration. The request to USPS and the response from USPS are displayed.
If the answer from USPS is empty, check that the parameter Proxy Server (only if necessary) is empty. This parameter Proxy Server is only necessary if your hosting provider requires a proxy server to make the CURL request to USPS (ie. Godaddy requires http://proxy.shr.secureserver.net:3128)