Zones shipping - shipping rate cannot be determined

Hi, I am using Zencart 1.38a. I have configured zones shipping where the calculation method = weight. The website has been live for three weeks now and I've used zones shipping since going live. I've successfully processed over 500 orders, except that for two orders the zones shipping has failed with the dreaded "Zone Rates (The shipping rate cannot be determined at this time)". I am using PayPal express and Payflow for payments and I've checked the PayPal logs, but everything looked OK, apart from the shipping cost wasn't included in the total order value. The correct country code and address was passed. I have checked the products that were ordered, and everything is correct, e.g., "Always Free Shipping" = "No, Normal Shipping Rules".

Are there any other logs apart from the PayPal logs that I can check to get more information about what caused the zones shipping failure? Is there anywhere where I can turn up debuging/logging?

Many thanks

P.S. - my customers and I are extremely impressed with the new Zencart website. Brilliant.