Hey there, @MotherofEve,
Sorry to hear your customers are running into this issue on your site!
When you say "their information", are you specifically referring to the customer's shipping address?
The reason I ask is that we've previously had similar reports from Sellers whose customers selected the option to manually enter an address, versus having it autofill, and the address was incorrectly formatted or missing information.
You'll notice when customers checkout, they have the option to autofill their address, or manually enter it. If they select the option to manually enter an address, they'll need to ensure they fill in every field as shown in my screenshots below.
If this is not the issue you're seeing, please include some additional information in your reply so I can take a closer look with you.
Example of Autofilled Address
Example of Manual Entry Address