Google Checkout's notification-failed messages are completely automated; no actual person has investigated anything when those messages are sent, so nobody is pointing any fingers from their end. All that message means is that nothing happened when their server attempted to contact ours, but that doesn't indicate the actual reason or source of the fault. Think of it this way: if your phone line is cut, we obviously won't be able to pick up when you dial our number, even though there is no problem with our own phone line.
We have personally investigated the issue thoroughly and run tests from our end and found nothing amiss, and we have contacted other e-commerce/cart providers who also integrate with Google Checkout, and they are also experiencing the exact, same sporadic pattern of errors with their Google Checkout callbacks lately, so obviously this issue is not isolated to E-junkie alone, indicating a general problem at Google's end affecting most or all Google Checkout merchants.