When the customer is checking out - on the freight selection page - they get a message saying something about unable to calculate freight or this is the only freight option available - with either no freight option visible or fewer than expected freight options (eg. perhaps they're close enough to your warehouse that they should have pick-up as an option as well as delivery - but no delivery option is visible, only pick-up)
The freight is configured in our freight tables. It's possible that the postcode range set for the customer isn't a range you have covered for in the domestic shipping tables in the CMS. Or if you are using the advanced shipping manager, the rules could be setup incorrectly. If you are unable to find the solution, please create a ticket and the support team will be able to assist you.
NOTE: if using Webninja's Second generation sync software - these settings are in the software. If using Webninja's current generation software and CMS - you'll need to edit the freight tables via the online CMS - if you're unsure which version you're using - please refer to these articles explaining the different versions of CMS and Software.