No Shipping Options

I upgraded from CS Cart 2 to 4.2.3 earlier this year, and apart from a few issues, it has been working OK and processing many 100's of orders a week.

However I have now had 4 customers phone and say that when they log in to their account and try to checkout, they get the “Unfortunately no shipping options are available for your location” message.

In each case, I have used the “act on behalf of” option, gone in to their account, and was able to check out no problem.

Just wondering if anyone else has had the same problem? I can't replicate it.