When the customer is checking out - on the freight selection page - they get a message saying something about unable to calculate fright 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)
Make sure that the customer has a post-code on the account. You can check this by looking at the customer in the CMS.
If the customer is checking out with delivery address details that have incorrect post-code details - it won't match up correctly to the corresponding post-code ranges set in the Freight tables. Go to the domestic shipping tables make sure that the customers postcode is in the range of a shipping table.
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 the following articles explaining the different versions of CMS and Sync Software.