USPS Shipping Marketpress

We set up all of the shipping options for USPS – selected United States as Target Country, selected Calculated Options, selected USPS, entered and selected all of the appropriate things under USPS settings – but if someone checks out they get the message: “No valid shipping options found. Please check your address carefully.” The address is fine. I had to set it to Flat Rate shipping for n ow just so orders won’t be lost, but any idea what would cause this?